object.c: use BUG(...) no die("BUG: ...") in lookup_object_by_type()

Adjust code added in 7463064b28 (object.h: add
lookup_object_by_type() function, 2021-06-22) to use the BUG()
function.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

object.c

@@ -199,7 +199,7 @@ struct object *lookup_object_by_type(struct repository *r,
 	case OBJ_BLOB:
 		return (struct object *)lookup_blob(r, oid);
 	default:
-		die("BUG: unknown object type %d", type);
+		BUG("unknown object type %d", type);
 	}
 }