Fix segfault when setting a device-pixel-ratio on a null-QPixmap

Change-Id: If3680766a50d5cf78889822d740c9472123191a7
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>

src/gui/image/qpixmap.cpp

@@ -678,6 +678,9 @@ qreal QPixmap::devicePixelRatio() const
 */
 void QPixmap::setDevicePixelRatio(qreal scaleFactor)
 {
+    if (isNull())
+        return;
+
     detach();
     data->setDevicePixelRatio(scaleFactor);
 }