gdesktopappinfo: Cancel file monitor when resetting a DesktopFileDir

It’s not enough to unref the monitor, since the GLib worker thread might
still hold a reference to it.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Helps: #1903

gio/gdesktopappinfo.c

@@ -1329,6 +1329,7 @@ desktop_file_dir_reset (DesktopFileDir *dir)
   if (dir->monitor)
     {
       g_signal_handlers_disconnect_by_func (dir->monitor, desktop_file_dir_changed, dir);
+      g_file_monitor_cancel (dir->monitor);
       g_object_unref (dir->monitor);
       dir->monitor = NULL;
     }