In QXcbWindow::setParent(), the window may not have a screen,
and in that case we cannot get the root window in this way.

Task-number: QTBUG-44719
Change-Id: I719e5e2f8cad13b1460b4d9df6ffd6c4a48e0d37
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>