Fix array-bounds warning in BVH

Provides a workaround to prevent tripping a compiler warning.

core/math/bvh_structs.inc

@@ -100,7 +100,11 @@ public:
 			num_items++;
 			return id;
 		}
+#ifdef DEV_ENABLED
 		return -1;
+#else
+		ERR_FAIL_V_MSG(0, "BVH request_item error.");
+#endif
 	}
 };