Null out relative_path_cache and relative_url_cache after we update

the pointers in case the hierarchy was adversely affected by actions
when the MPTT pointers were desynced.  Fixes ticket #1235.

modules/gallery/helpers/gallery_task.php

@@ -381,6 +381,8 @@ static function fix_mptt_set_right($id, $value) {
     db::build()
       ->update("items")
       ->set("right_ptr", $value)
+      ->set("relative_path_cache", null)
+      ->set("relative_url_cache", null)
       ->where("id", "=", $id)
       ->execute();
   }