message: fix issue 8189.
diff: @@ -1395,7 +1395,7 @@ void _InputArray::getMatVector(std::vector<Mat>& mv) const
  {
  const std::vector<uchar>& v = *(const std::vector<uchar>*)obj;
  - size_t n = v.size(), esz = CV_ELEM_SIZE(flags);
  + size_t n = size().width, esz = CV_ELEM_SIZE(flags);
  int t = CV_MAT_DEPTH(flags), cn = CV_MAT_CN(flags);
  mv.resize(n);
  