[server] Don't crash when a subsurface gets commited whose parent surface got destroyed
Qt seems to damage and commit child subsurfaces although their parent
got destroyed. This actually doesn't make any sense as without a parent
surface they cannot be shown. But nevertheless we should not crash in
such a situation.
This change guards the places in the commit handling code where the
parent gets accessed.
Test Plan: New test case which exposes the problem
Differential Revision: https://phabricator.kde.org/D10300