Prevent segfault if repair item disappears (CleverRaven#75413)

Prevents segfault that could previously occur if the item being repaired
was invalidated. The target item being repaired could get invalidated
for various reasons, such as a follower moving it via zone sorting, the
item could burn up in a fire, etc.

Previous segfault that this commit attempts to prevent:
```
 #0  0x0000555555ef9057 in item::is_null (this=this@entry=0x0) at src/item.cpp:943
 #1  0x000055555602322c in repair_item_actor::can_repair_target (this=this@entry=0x555560003e80, pl=..., fix=..., print_msg=print_msg@entry=true, check_consumed_available=check_consumed_available@entry=true) at src/iuse_actor.cpp:2920
 #2  0x00005555560243c2 in repair_item_actor::repair (this=this@entry=0x555560003e80, pl=..., tool=..., fix=..., refit_only=refit_only@entry=false) at src/iuse_actor.cpp:3106
 #3  0x00005555559d5b55 in repair_item_finish (act=0x5555580cdf58, you=0x5555580cd890, no_menu=false) at src/activity_handlers.cpp:2477
 #4  0x0000555555a00f52 in std::function<void (player_activity*, Character*)>::operator()(player_activity*, Character*) const (this=<optimized out>, __args#0=<optimized out>, __args#0@entry=0x5555580cdf58, __args#1=<optimized out>, __args#1@entry=0x5555580cd890) at /usr/include/c++/13/bits/std_function.h:591
 #5  0x00005555559ff229 in activity_type::call_finish (this=<optimized out>, act=act@entry=0x5555580cdf58, you=you@entry=0x5555580cd890) at src/activity_type.cpp:168
 CleverRaven#6  0x00005555564e368b in player_activity::do_turn (this=0x5555580cdf58, you=...) at src/player_activity.cpp:393
 CleverRaven#7  0x0000555555cec856 in do_turn () at src/do_turn.cpp:532
 CleverRaven#8  0x0000555555776e74 in main (argc=<optimized out>, argv=<optimized out>) at src/main.cpp:873
```

Instead, this commit adds a message shown to the player if the target
item is no longer valid for whatever reason.

src/activity_handlers.cpp

@@ -2471,6 +2471,12 @@ void repair_item_finish( player_activity *act, Character *you, bool no_menu )
     // Valid Repeat choice and target, attempt repair.
     if( repeat != repeat_type::INIT && act->targets.size() >= 2 ) {
         item_location &fix_location = act->targets[1];
+        if( !fix_location ) {
+            // The item could disappear for various reasons: moved by follower, burned up, eaten by a grue, etc.
+            you->add_msg_if_player( m_warning, _( "You can no longer find the item to repair." ) );
+            act->set_to_null();
+            return;
+        }
 
         // Remember our level: we want to stop retrying on level up
         const int old_level = you->get_skill_level( actor->used_skill );