filters-manager: fix leak of sidecar task

Part-of: <https://gitlab.gnome.org/GNOME/epiphany/-/merge_requests/958>


(cherry picked from commit 77a128cda3abfebf5b998dfbbd5e4bc584f1246c)

embed/ephy-filters-manager.c

@@ -230,6 +230,7 @@ sidecar_bytes_loaded_cb (GFile        *file,
   } else {
     g_task_return_error (task, error);
   }
+  g_object_unref (task);
 }
 
 static void