Avoid crash without XWayland
Summary: readX11ShadowProperty fails without an X connection
Test Plan: Tested by Kai who didn't have xwayland
Reviewers: KWin, zzag
Reviewed By: KWin, zzag
Subscribers: zzag, kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D21965