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
(cherry picked from commit 52fdd24467fa8d6f97bd5f9eb6d5509fa43436c6)
ca0d56a3a4