Fix the bug of Mat_<>::opeartor []

`template<typename _Tp> inline const _Tp* Mat_<_Tp>::operator [](int y) const` does not support 3d matrix since it checks rows.

This operator[] shall check size.p[0] instead.

modules/core/include/opencv2/core/mat.inl.hpp

@@ -1634,14 +1634,14 @@ Mat_<_Tp> Mat_<_Tp>::operator()(const std::vector<Range>& ranges) const
 template<typename _Tp> inline
 _Tp* Mat_<_Tp>::operator [](int y)
 {
-    CV_DbgAssert( 0 <= y && y < rows );
+    CV_DbgAssert( 0 <= y && y < size.p[0] );
     return (_Tp*)(data + y*step.p[0]);
 }
 
 template<typename _Tp> inline
 const _Tp* Mat_<_Tp>::operator [](int y) const
 {
-    CV_DbgAssert( 0 <= y && y < rows );
+    CV_DbgAssert( 0 <= y && y < size.p[0] );
     return (const _Tp*)(data + y*step.p[0]);
 }