Prevent segfault on unload_activity_actor

Prevents segfault that previously happened when pouring liquid to ground
from AIM using "examine" menu.

Previous segfault being fixed:
```
 #0  __gnu_cxx::__atomic_add_dispatch () at /usr/include/c++/13/ext/atomicity.h:111
 #1  std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_add_ref_copy () at /usr/include/c++/13/bits/shared_ptr_base.h:152
 #2  0x00005652b2df5540 in std::shared_ptr<item_location::impl>::shared_ptr () at /usr/include/c++/13/bits/shared_ptr.h:204
 #3  item_location::item_location () at src/item_location.h:30
 #4  item_location::impl::item_in_container::item_in_container () at src/item_location.cpp:612
 #5  item_location::item_location () at src/item_location.cpp:767
 #6  0x00005652b27fa6b1 in unload_activity_actor::unload () at src/activity_actor.cpp:3232
 #7  0x00005652b32eed69 in player_activity::do_turn () at src/player_activity.cpp:383
 #8  0x00005652b2b553e7 in do_turn () at src/do_turn.cpp:487
 #9  0x00005652b25ea589 in main () at src/main.cpp:798
```

The problem before was that the reference to `target` in
`unload_activity_actor::unload` was an invalid reference. This was
caused by:
  1. `unload_activity_actor::finish` calls `act.set_to_null()`.
  2. That sets the player activity to `null` type (=no current activity)
  3. `unload_activity_actor::unload` is `static`, and takes `target` as
     a reference.
  4. `::unload` calls `Character::add_or_drop_with_msg` which leads to
     call chain: `liquid_handler::consume_liquid` -> `get_liquid_target`
     -> `choose_adjacent` -> `choose_direction`
     -> `temp_hide_advanced_inv` -> `advanced_inventory::temp_hide`
     -> `advanced_inventory::do_return_entry`
  5. `advanced_inventory::do_return_entry` assigns a new
     `ACT_ADV_INVENTORY` activity. This invalidates the previous
     `unload_activity_actor` because of pt 2 above.
  6. When static method `unload_activity_actor::unload` resumes after
     its call to `add_or_drop_with_msg`, the reference to `target` is
     invalid because of pt 5.

This commit attempt to fix the issue with invalidated `target` reference
by copying its value before invalidating the activity.

src/activity_actor.cpp

@@ -3204,8 +3204,11 @@ void unload_activity_actor::start( player_activity &act, Character & )
 
 void unload_activity_actor::finish( player_activity &act, Character &who )
 {
+    // Need to copy `target` here because `::unload` is static and `this` might be invalidated
+    // after `act.set_to_null()` has been called (so `this.target` might also be invalidated)
+    item_location target_copy = target;
     act.set_to_null();
-    unload( who, target );
+    unload( who, target_copy );
 }
 
 void unload_activity_actor::unload( Character &who, item_location &target )