fix crash in QWidgetPrivate::screenGeometry
fix crash in QWidgetPrivate::screenGeometry

Missing null check added. Not really a Windows CE only crash.
The following code crashed on all platforms, because the
QGraphicsProxyWidget had no scene yet.

QPushButton *button = new QPushButton;
QGraphicsProxyWidget *buttonProxy = new QGraphicsProxyWidget;
buttonProxy->setWidget(button);
qApp->desktop()->screenGeometry(button);

Task-number: QTBUG-5626
Reviewed-by: Alessandro Portale

radekp/qt,radekp/qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,radekp/qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,radekp/qt,radekp/qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,radekp/qt,radekp/qt,radekp/qt,pruiz/wkhtmltopdf-qt,radekp/qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,radekp/qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk