Fixed crash when resizing a scroll area with a native viewport
(typically a QGLWidget) causes the scroll bars to get hidden, while
there are still expose events in the queue for the scroll bar widget.
Task-number: QTBUG-26746
Change-Id: Ia77c8eb32a6730670333120af3f9a772c64807f1
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>