[server] Don't crash when a subsurface gets commited whose parent surface got destroyed

Summary:
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.

BUG: 389231

Test Plan: New test case which exposes the problem

Reviewers: #frameworks, #kwin, #plasma

Subscribers: plasma-devel

Tags: #plasma, #frameworks

Differential Revision: https://phabricator.kde.org/D10300