[mutter] wayland: avoid a crash in is_effectively_synchronized()

[Olivier Fourdan <ofourdan src gnome org>]

commit 52fdd24467fa8d6f97bd5f9eb6d5509fa43436c6
Author: Olivier Fourdan <ofourdan redhat com>
Date:   Thu May 3 09:17:24 2018 +0200

    wayland: avoid a crash in is_effectively_synchronized()
    
    To check if a subsurface is effectively synchronized, we walk the
    subsurface hierarchy to look for a non-subsurface parent or a subsurface
    being synchronized.
    
    However, when client is closing, the parent surface might already be
    gone, in which case we end up with a surface being NULL which causes a
    NULL pointer dereference and a crash.
    
    Check if the parent surface is NULL to avoid the crash, and consider
    it's already synchronized if it is NULL to avoid further updates.
    
    Fixes: https://gitlab.gnome.org/GNOME/mutter/issues/124

 src/wayland/meta-wayland-surface.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
---
diff --git a/src/wayland/meta-wayland-surface.c b/src/wayland/meta-wayland-surface.c
index c0cef357a..0142e1a3d 100644
--- a/src/wayland/meta-wayland-surface.c
+++ b/src/wayland/meta-wayland-surface.c
@@ -573,7 +573,10 @@ meta_wayland_surface_is_effectively_synchronized (MetaWaylandSurface *surface)
         {
           MetaWaylandSurface *parent = surface->sub.parent;
 
-          return meta_wayland_surface_is_effectively_synchronized (parent);
+          if (parent)
+            return meta_wayland_surface_is_effectively_synchronized (parent);
+
+          return TRUE;
         }
     }
 }