message: core: handle empty Mat in Mat_ assignment operators
diff: @@ -1744,6 +1744,11 @@ Mat_<_Tp>::Mat_(const std::array<_Tp, _Nm>& arr, bool copyData)
  template<typename _Tp> inline
  Mat_<_Tp>& Mat_<_Tp>::operator = (const Mat& m)
  {
  + if (m.empty())
  + {
  + release();
  + return *this;
  + }
  if( traits::Type<_Tp>::value == m.type() )
  {
  Mat::operator = (m);
  @@ -2125,6 +2130,11 @@ Mat_<_Tp>::Mat_(Mat&& m)
  template<typename _Tp> inline
  Mat_<_Tp>& Mat_<_Tp>::operator = (Mat&& m)
  {
  + if (m.empty())
  + {
  + release();
  + return *this;
  + }
  if( traits::Type<_Tp>::value == m.type() )
  {
  Mat::operator = ((Mat&&)m);
  