ktooltipwidget: Prevent offscreen display

Summary:
Try to fix test failure in CI
https://build.kde.org/view/Frameworks/job/Frameworks%20kwidgetsaddons%20kf5-qt5%20SUSEQt5.10/

Reviewers: elvisangelaccio, cfeck

Reviewed By: cfeck

Subscribers: cfeck, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D11677