Prevent segfault if repair item disappears

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
 CleverRaven#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
 CleverRaven#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
 CleverRaven#3  0x00005555559d5b55 in repair_item_finish (act=0x5555580cdf58, you=0x5555580cd890, no_menu=false) at src/activity_handlers.cpp:2477
 CleverRaven#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
 CleverRaven#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item is no longer valid for whatever reason.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 );