fix #962: the image of the previously rendered feature was used inste…

…ad of an empty one.

git-svn-id: http://svn.osgeo.org/qgis/trunk/qgis@8294 c8812cc2-4d05-0410-92ff-de0c093fc19c

src/core/renderer/qgscontinuouscolorrenderer.cpp

@@ -80,7 +80,11 @@ void QgsContinuousColorRenderer::renderFeature(QPainter * p, QgsFeature & f, QIm
     //first find out the value for the classification attribute
     const QgsAttributeMap& attrs = f.attributeMap();
     if( attrs[mClassificationField].isNull() )
+    {
+      if(img)
+        *img = QImage();
       return;
+    }
     double fvalue = attrs[mClassificationField].toDouble();
 
     //double fvalue = vec[mClassificationField].fieldValue().toDouble();