tr2/objects/harpoon_bolt: fix harpoon test on invalid objects

This fixes testing harpoon colliding with invalid objects, such as
sprite pickups.

Resolves #2718.

src/tr2/game/objects/general/harpoon_bolt.c

@@ -62,6 +62,9 @@ static void M_Control(const int16_t item_num)
         }
 
         const ANIM_FRAME *const frame = Item_GetBestFrame(target_item);
+        if (frame == nullptr) {
+            continue;
+        }
         const BOUNDS_16 *const bounds = &frame->bounds;
 
         const int32_t cdy = item->pos.y - target_item->pos.y;