Remove warning about non existing paint device engine

Remove warning about non existing paint device engine

The QWindowsVistaStyle::drawPrimitive() method was trying to draw the
pixmap for an item in an item view even if the section width was zero
or less then zero. This resulted "QPainter::begin: Paint device
returned engine == 0, type: 2" error.
It's now checked that the width and the height of the section is
greater than zero before creating the painter.

Task-number: QTBUG-26047
Change-Id: I36dd2792fd505be773951e5ac36b0e3362ec2292
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>