inject: fix item creation

This uses the correct approach to create an item on the fly. The issue
itself was unreleased, and the change in c279b4e brought the bug to
light.

Resolves LostArtefacts#1266.

src/game/inject.c

@@ -1277,7 +1277,9 @@ static void Inject_TriggeredItem(INJECTION *injection, LEVEL_INFO *level_info)
         return;
     }
 
-    ITEM_INFO *item = &g_Items[g_LevelItemCount];
+    int16_t item_number = Item_Create();
+    ITEM_INFO *item = &g_Items[item_number];
+
     File_Read(&item->object_number, sizeof(int16_t), 1, fp);
     File_Read(&item->room_number, sizeof(int16_t), 1, fp);
     File_Read(&item->pos.x, sizeof(int32_t), 1, fp);