Only cleanup XRenderUtils is we actually created it

Summary: BUG: 386430

Test Plan:
kwin_x11 --crashes 20
Got a dialog. Hit cancel. It didn't crash

Reviewers: #plasma, graesslin

Reviewed By: #plasma, graesslin

Subscribers: plasma-devel, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8593