message: add !empty assertion in seamlessClone()
  issue addresses lack of warnings on
  seamlessClone() function when src is None.
  This commit adds source check using CV_Assert
  therefore debugging would be easier.
diff: @@ -67,6 +67,7 @@ static Mat checkMask(InputArray _mask, Size size)
  void cv::seamlessClone(InputArray _src, InputArray _dst, InputArray _mask, Point p, OutputArray _blend, int flags)
  {
  CV_INSTRUMENT_REGION();
  + CV_Assert(!_src.empty());
  const Mat src = _src.getMat();
  const Mat dest = _dst.getMat();
  