Fix a missing assertion

game/map/game_map_entities.c

@@ -63,6 +63,7 @@ void map_game_map_npc_set_inflict_id(sys_i32 id) {
 }
 
 void map_game_map_npc_set_face_left(bool face_left) {
+    assert(game_map_npc_in_progress && "NPC has to be in progress");
     game_map_npc_in_progress->face_left = face_left;
 }