Prevent crash due to giving QWidget::update() a large rect.

We can simply clip the update rect against the widget's rect and return
if it's empty. Otherwise we risk ending up with update rects that are
larger than INT_MAX due to multiple update rects being merged.

Task-number: QTBUG-30876
Change-Id: I23bd0149fbe8d1a007a60b228e6bddb45dc4fc32
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>