message: fix medianBlur accessviolation
  medianBlur called with "empty" source and ksize >= 7 crashes application with accessviolation. With this extra assert this is avoided and the application may normally catch the thrown exception.
diff: @@ -3494,7 +3494,7 @@ void cv::medianBlur( InputArray _src0, OutputArray _dst, int ksize )
  CV_Assert( (ksize % 2 == 1) && (_src0.dims() <= 2 ));
  - if( ksize <= 1 )
  + if( ksize <= 1 || _src0.empty() )
  {
  _src0.copyTo(_dst);
  return;
  