wayland: Plug XDnD drag source leak

This object is just being detached, with no code unref'ing it. Do
this whenever the XDnD selection goes unowned, usually a good
indication that the drag source no longer is one.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1720>

src/wayland/meta-xwayland-dnd.c

@@ -909,6 +909,7 @@ meta_xwayland_dnd_handle_xfixes_selection_notify (MetaWaylandCompositor *composi
   else if (event->owner == None)
     {
       meta_xwayland_end_dnd_grab (data_device, FALSE);
+      g_clear_object (&dnd->source);
     }
 
   return FALSE;