Prevent using invalidated explosion data (CleverRaven#75711)

Prevents crashes that previously happened from using references to
`explosion_data` that had been invalidated.

The problem before happened when killing "unfolded impossibility" in
LIXA facility with a grenade, since this monster runs an eoc that
switches maps. That led to `explosion_handler::process_explosions` being
called twice recursively. First invocation iterates references, and the
second invocation might append to the vector, and most certaily will
clear the vector, thus invalidating references for the first invocation.

Example crash being fixed by this commit, notice how
`explosion_handler::process_explosions` occurs twice in the callstack:

```
Thread 1 "cataclysm-tiles" received signal SIGABRT, Aborted.
__pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at ./nptl/pthread_kill.c:44

(gdb) bt
 #0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at ./nptl/pthread_kill.c:44
 #1  0x00007ffff787840f in __pthread_kill_internal (signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:78
 #2  0x00007ffff78294f2 in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
 #3  0x00007ffff78124ed in __GI_abort () at ./stdlib/abort.c:79
 #4  0x00007ffff7ad501e in std::__glibcxx_assert_fail(char const*, int, char const*, char const*) () from /lib/x86_64-linux-gnu/libstdc++.so.6
 #5  0x000055555688b471 in std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const*, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const*> >::operator[] (this=<optimized out>, __n=1967424379) at /usr/include/c++/14/bits/stl_vector.h:1128
 #6  std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const*, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const*> >::operator[] (this=<optimized out>, __n=1967424379) at /usr/include/c++/14/bits/stl_vector.h:1128
 #7  string_identity_static::get_interned_string[abi:cxx11](int) (id=1967424379) at src/string_id.cpp:51
 #8  0x0000555555fffed7 in string_identity_static::str[abi:cxx11]() const (this=<optimized out>) at src/string_id.h:140
 CleverRaven#9  0x0000555556057423 in string_id<itype>::c_str (this=this@entry=0x5555a126a718) at src/string_id.h:253
 CleverRaven#10 0x00005555560be56a in Item_factory::find_template (this=0x55555751cad0, id=...) at src/item_factory.cpp:2563
 CleverRaven#11 0x0000555555e4b0c5 in explosion_handler::_make_explosion (source=<optimized out>, p=..., ex=...) at /usr/include/c++/14/bits/unique_ptr.h:193
 CleverRaven#12 0x0000555555e4b704 in explosion_handler::process_explosions () at src/explosion.cpp:923
 CleverRaven#13 0x0000555556246e97 in map::actualize (this=this@entry=0x5555a12bf890, grid=...) at src/map.cpp:9168
 CleverRaven#14 0x00005555562472c0 in map::load (this=this@entry=0x5555a12bf890, w=..., update_vehicle=update_vehicle@entry=true, pump_events=pump_events@entry=false) at src/map.cpp:8387
 CleverRaven#15 0x0000555555de2546 in tinymap::load (this=this@entry=0x5555a12bf890, w=..., update_vehicles=update_vehicles@entry=true, pump_events=pump_events@entry=false) at src/map.h:2765
 CleverRaven#16 0x00005555562a7e6f in update_mapgen_function_json::update_map (this=0x55555a8ce8f0, omt_pos=..., args=..., offset=..., miss=miss@entry=0x0, verify=verify@entry=true, mirror_horizontal=false, mirror_vertical=false, rotation=0) at src/mapgen.cpp:8015
 CleverRaven#17 0x00005555562a81e3 in run_mapgen_update_func (update_mapgen_id=..., omt_pos=..., args=..., miss=miss@entry=0x0, cancel_on_collision=cancel_on_collision@entry=true, mirror_horizontal=mirror_horizontal@entry=false, mirror_vertical=false, rotation=0) at src/mapgen.cpp:8117
 CleverRaven#18 0x00005555565a0250 in operator() (__closure=<optimized out>, d=...) at src/npctalk.cpp:3964
 CleverRaven#19 0x000055555659c7a4 in talk_effect_t::apply (this=this@entry=0x55555d5211d8, d=...) at src/npctalk.cpp:6526
 CleverRaven#20 0x0000555555e0c9ba in effect_on_condition::activate (this=0x55555d521160, d=..., require_callstack_check=require_callstack_check@entry=true) at src/effect_on_condition.cpp:329
 CleverRaven#21 0x00005555561f9693 in spell_effect::effect_on_condition (sp=..., caster=..., target=...) at src/magic_spell_effect.cpp:1806
 CleverRaven#22 0x00005555561d101e in spell::cast_all_effects (this=0x7fffffffc788, source=..., target=...) at src/magic.cpp:1907
 CleverRaven#23 0x0000555556463bec in monster::die (this=0x55559c7fc0a0, nkiller=0x0) at src/monster.cpp:2941
 CleverRaven#24 0x0000555555d33386 in Creature::deal_projectile_attack (this=0x55559c7fc0a0, source=0x0, attack=..., print_messages=<optimized out>, wp_attack=...) at src/creature.cpp:1311
 CleverRaven#25 0x0000555556465049 in monster::deal_projectile_attack (this=this@entry=0x55559c7fc0a0, source=source@entry=0x0, attack=..., print_messages=print_messages@entry=false, wp_attack=...) at src/monster.cpp:2212
 CleverRaven#26 0x0000555555e4a9d9 in explosion_handler::shrapnel (range=-1, source=<optimized out>, src=..., power=<optimized out>, casing_mass=<optimized out>, per_fragment_mass=<optimized out>) at src/explosion.cpp:463
 CleverRaven#27 explosion_handler::_make_explosion (source=<optimized out>, p=..., ex=...) at src/explosion.cpp:536
 CleverRaven#28 0x0000555555e4b704 in explosion_handler::process_explosions () at src/explosion.cpp:923
 CleverRaven#29 0x0000555555dcf46f in do_turn () at src/do_turn.cpp:648
 CleverRaven#30 0x00005555557a1227 in main (argc=<optimized out>, argv=<optimized out>) at src/main.cpp:873
```

In the crash above, the `explosion_data` has been invalidated:
```
(gdb) frame 11
193           pointer    _M_ptr() const noexcept { return std::get<0>(_M_t); }

(gdb) print ex
$1 = (const explosion_data &) @0x5555a126a6f8: {power = 1.75295132e+25, distance_factor = 2.76847299e+20, max_noise = 1667855474, fire = 117, shrapnel = {casing_mass = 1936026889, fragment_mass = 6.82915174e+22, recovery = -1584519120, drop = {_version = 13059389229367304, _cid = 2019155690, _id = {
        _id = 1967424379}}}}

(gdb) print ex.shrapnel.drop
$2 = {_version = 13059389229367304, _cid = 2019155690, _id = {_id = 1967424379}}
```

src/explosion.cpp

@@ -911,15 +911,29 @@ void resonance_cascade( const tripoint &p )
 
 void process_explosions()
 {
-    for( const queued_explosion &ex : _explosions ) {
+    if( _explosions.empty() ) {
+        return;
+    }
+
+    // Need to copy and clear this vector before processing the explosions.
+    // Part of processing in `_make_explosion` is handing out shrapnel damage,
+    // which might kill monsters. That might have all sorts of consequences,
+    // such as running eocs, loading new maps (via eoc) or other explosions
+    // being added. There is therefore a chance that we might recursively
+    // enter this function again during explosion processing, and we need to
+    // guard against references becoming invalidated either by items being
+    // added to the vector, or us clearing it here.
+    std::vector<queued_explosion> explosions_copy( _explosions );
+    _explosions.clear();
+
+    for( const queued_explosion &ex : explosions_copy ) {
         const tripoint p = get_map().getlocal( ex.pos );
         if( p.x < 0 || p.x >= MAPSIZE_X || p.y < 0 || p.y >= MAPSIZE_Y ) {
             debugmsg( "Explosion origin (%d, %d, %d) is out-of-bounds", p.x, p.y, p.z );
             continue;
         }
         _make_explosion( ex.source, p, ex.data );
     }
-    _explosions.clear();
 }
 
 } // namespace explosion_handler