before marking the backingstore as dirty. The QWidgetBackingstore has an
assert that the region or the rect passed in is not empty, hence its
programming error not to check before calling this function.

Task-number: QTBUG-43489
Change-Id: Ic67fb6ca7959466e1758ce91827cd4b8acdf73fc
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>