surface-actor-wayland: Handle Xwayland vanishing gracefully

When a game is playing fullscreen, mutter may use a direct scanout of
the surface.

However, if that surface is from Xwayland and Xwayland had vanished (now
that mutter can survive that), the actual surface might be NULL, so
check for that case to avoid a crash.

Close: https://gitlab.gnome.org/GNOME/mutter/-/issues/1638
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1718>

src/compositor/meta-surface-actor-wayland.c

@@ -74,6 +74,9 @@ meta_surface_actor_wayland_try_acquire_scanout (MetaSurfaceActorWayland *self,
   CoglScanout *scanout;
 
   surface = meta_surface_actor_wayland_get_surface (self);
+  if (!surface)
+    return NULL;
+
   scanout = meta_wayland_surface_try_acquire_scanout (surface, onscreen);
   if (!scanout)
     return NULL;