Prevent segfault when deconstructing improvised shelter

Previous segfault happened when trying to display the list of items that
deconstruction would yield, but this terrain has no `deconstruct` json.

This commit also adds a check with `debugmsg` for future such cases
during the game startup validation.

Previous crash being fixed:
```
Thread 1 "cataclysm-tiles" received signal SIGSEGV, Segmentation fault.
0x0000555555cb9602 in operator() (drop_group=..., __closure=<optimized out>) at src/construction.cpp:2049
2049                            item_group::spawn_data_from_group( drop_group )->every_item_min_max();
(gdb) bt
 #0  0x0000555555cb9602 in operator() (drop_group=..., __closure=<optimized out>) at src/construction.cpp:2049
 #1  0x0000555555cb9af2 in construct::do_turn_deconstruct (p=..., who=...) at src/construction.cpp:2086
 #2  0x0000555555a32fad in activity_handlers::build_do_turn (act=0x5555583e4338, you=0x5555583e3c70) at src/activity_handlers.cpp:3424
 #3  0x0000555555a6b7de in std::function<void(player_activity*, Character*)>::operator() (this=<optimized out>, __args#0=<optimized out>, __args#0@entry=0x5555583e4338, __args#1=<optimized out>, __args#1@entry=0x5555583e3c70) at /usr/include/c++/14/bits/std_function.h:591
 #4  0x0000555555a69647 in activity_type::call_do_turn (this=<optimized out>, act=act@entry=0x5555583e4338, you=you@entry=0x5555583e3c70) at src/activity_type.cpp:160
 #5  0x00005555566d3a1c in player_activity::do_turn (this=0x5555583e4338, you=...) at src/player_activity.cpp:320
 #6  0x0000555555dcef82 in do_turn () at src/do_turn.cpp:592
 #7  0x00005555557a1217 in main (argc=<optimized out>, argv=<optimized out>) at src/main.cpp:873
```

data/json/furniture_and_terrain/LIXA_furniture_and_terrain.json

@@ -10,7 +10,7 @@
     "light_emitted": 120,
     "looks_like": "t_carpet_green",
     "connect_groups": "INDOORFLOOR",
-    "flags": [ "TRANSPARENT", "FLAMMABLE_HARD", "SUPPORTS_ROOF", "COLLAPSES", "INDOORS", "FLAT", "RUG", "EASY_DECONSTRUCT" ],
+    "flags": [ "TRANSPARENT", "FLAMMABLE_HARD", "SUPPORTS_ROOF", "COLLAPSES", "INDOORS", "FLAT", "RUG" ],
     "bash": {
       "str_min": 4,
       "str_max": 12,

data/json/furniture_and_terrain/terrain-manufactured.json

@@ -1376,6 +1376,7 @@
       "sound_fail": "whump!",
       "items": [ { "item": "rock_large", "count": [ 0, 20 ] }, { "item": "sharp_rock", "count": [ 0, 16 ] } ]
     },
+    "deconstruct": { "ter_set": "t_dirt", "items": [ { "item": "rock_large", "count": 22 } ] },
     "flags": [ "TRANSPARENT", "THIN_OBSTACLE", "SHORT", "ROUGH", "UNSTABLE", "PERMEABLE", "EASY_DECONSTRUCT" ]
   },
   {
@@ -1494,6 +1495,7 @@
       "HIDE_PLACE",
       "EASY_DECONSTRUCT"
     ],
+    "deconstruct": { "ter_set": "t_pit_shallow", "items": [ { "item": "stick", "count": 12 }, { "item": "pine_bough", "count": 24 } ] },
     "bash": {
       "str_min": 4,
       "str_max": 60,
@@ -1526,6 +1528,10 @@
       "HIDE_PLACE",
       "EASY_DECONSTRUCT"
     ],
+    "deconstruct": {
+      "ter_set": "t_pit_shallow",
+      "items": [ { "item": "stick", "count": 12 }, { "item": "pine_bough", "count": 24 }, { "item": "withered", "count": 80 } ]
+    },
     "bash": {
       "str_min": 4,
       "str_max": 60,

src/construction.cpp

@@ -2045,8 +2045,12 @@ void construct::do_turn_deconstruct( const tripoint_bub_ms &p, Character &who )
 
         auto deconstruct_items = []( const item_group_id & drop_group ) {
             std::string ret;
+            const Item_spawn_data *spawn_data = item_group::spawn_data_from_group( drop_group );
+            if( spawn_data == nullptr ) {
+                return ret;
+            }
             const std::map<const itype *, std::pair<int, int>> deconstruct_items =
-                        item_group::spawn_data_from_group( drop_group )->every_item_min_max();
+                        spawn_data->every_item_min_max();
             for( const auto &deconstruct_item : deconstruct_items ) {
                 const int &min = deconstruct_item.second.first;
                 const int &max = deconstruct_item.second.second;

src/mapdata.cpp

@@ -1039,6 +1039,10 @@ void ter_t::check() const
             debugmsg( "ter %s has invalid emission %s set", id.c_str(), e.str().c_str() );
         }
     }
+    if( has_flag( ter_furn_flag::TFLAG_EASY_DECONSTRUCT ) && !deconstruct.can_do ) {
+        debugmsg( "ter %s has EASY_DECONSTRUCT flag but cannot be deconstructed",
+                  id.c_str(), deconstruct.drop_group.c_str() );
+    }
 }
 
 furn_t::furn_t() : open( furn_str_id::NULL_ID() ), close( furn_str_id::NULL_ID() ) {}