decorationbridge: Fix crash on plasma mobile

Summary:
For some reason there m_settings is null there, don't crash in this case.
The code definitely contemplates the possibility since it's null by default and on some DecorationBridge::reconfigure() paths.

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: zzag, anthonyfieroni, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D27809