wayland/subsurface: Handle node unlinking on parent destroyed

It concerns subsurface state and the subsurface already listenes
for the parent to get destroyed.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1958>

src/wayland/meta-wayland-subsurface.c

@@ -502,6 +502,7 @@ surface_handle_parent_surface_destroyed (struct wl_listener *listener,
                                                  surface,
                                                  sub.parent_destroy_listener);
 
+  g_node_unlink (surface->subsurface_branch_node);
   surface->sub.parent = NULL;
 }