Fix crash on null Qimage in QSGPainterNode::paint

Creating a QPainter on a null image is not allowed,
return early if this is the case.

Change-Id: I56f30c0fd84eefc1676d6b980f77269abace07ce
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>

src/quick/scenegraph/util/qsgpainternode.cpp

@@ -154,6 +154,9 @@ void QSGPainterNode::paint()
 {
     QRect dirtyRect = m_dirtyRect.isNull() ? QRect(0, 0, m_size.width(), m_size.height()) : m_dirtyRect;
 
+    if (m_image.isNull())
+        return;
+
     QPainter painter;
     if (m_actualRenderTarget == QQuickPaintedItem::Image)
         painter.begin(&m_image);