wayland/xdg-shell: Ignore reposition if popup was dismissed

If the popup was dismissed (i.e. has no MetaWindow anymore), it'll also
have no parent surface. With no parent surface, we'd try to fetch a
transaction from NULL and crash, but if we don't try if we were
dismissed, we won't reach here anyway.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2940>

src/wayland/meta-wayland-xdg-shell.c

@@ -642,6 +642,9 @@ xdg_popup_reposition (struct wl_client   *client,
   MetaWaylandXdgPositioner *xdg_positioner;
   MetaWaylandTransaction *transaction;
 
+  if (!meta_wayland_surface_get_window (surface))
+    return;
+
   xdg_positioner = g_memdup2 (wl_resource_get_user_data (positioner_resource),
                               sizeof (MetaWaylandXdgPositioner));