wayland/surface: Reset buffer_destroy_handler_id

Syncronized subsurfaces that call into `merge_pending_state` might
otherwise not create new destroy handlers, ending up with a invalid
handler ids, throwing errors and leaking.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/868

src/wayland/meta-wayland-surface.c

@@ -401,7 +401,7 @@ static void
 pending_buffer_resource_destroyed (MetaWaylandBuffer       *buffer,
                                    MetaWaylandPendingState *pending)
 {
-  g_signal_handler_disconnect (buffer, pending->buffer_destroy_handler_id);
+  g_clear_signal_handler (&pending->buffer_destroy_handler_id, buffer);
   pending->buffer = NULL;
 }