Guard resource deletion in OuptutConfiguration::sendApplied

Summary:
A client (kscreen-doctor especially) can disconnect whilst kwin still
has an instance.

Test Plan:
Had reproducible crash on kscreen-doctor.
Now don't.

Reviewers: KWin

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D18248