[wayland] Finish active move-resize op when client is destroyed or unmapped

Summary:
It might happen that the moving client gets closed or crashes. In that
case, we have to manually reset Workspace::movingClient, otherwise KWin
will most likely crash later on.

BUG: 405379

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D19708