wayland: Skip subsurface desync if parent is NULL

In order to avoid crashes when a client already destroyed the parent
wl_surface.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2917>

(cherry picked from commit 76ce6a0ab5975062ffe1f8b885b9be650b60e5a7)

1 files changed, 4 insertions, 2 deletions

diff --git a/src/wayland/meta-wayland-subsurface.c b/src/wayland/meta-wayland-subsurface.c
index 53c35c5dc..d141483be 100644
--- a/src/wayland/meta-wayland-subsurface.c
+++ b/src/wayland/meta-wayland-subsurface.c
@@ -466,13 +466,15 @@ wl_subsurface_set_desync (struct wl_client   *client,
                           struct wl_resource *resource)
 {
   MetaWaylandSurface *surface = wl_resource_get_user_data (resource);
+  MetaWaylandSurface *parent;
   gboolean is_parent_effectively_synchronized;
 
-  if (!surface->sub.synchronous)
+  parent = surface->sub.parent;
+  if (!parent || !surface->sub.synchronous)
     return;
 
   is_parent_effectively_synchronized =
-    is_surface_effectively_synchronized (surface->sub.parent);
+    is_surface_effectively_synchronized (parent);
 
   if (!is_parent_effectively_synchronized)
     meta_wayland_surface_apply_cached_state (surface);