Merge pull request #16992 from afowles:fix-forEach-segfault

* Fixed divide by zero error in forEach

* Dedicated assertion for !empty

modules/core/include/opencv2/core/utility.hpp

@@ -622,6 +622,7 @@ void Mat::forEach_impl(const Functor& operation) {
         //  or (_Tp&, void*)        <- in case you don't need current idx.
     }
 
+    CV_Assert(!empty());
     CV_Assert(this->total() / this->size[this->dims - 1] <= INT_MAX);
     const int LINES = static_cast<int>(this->total() / this->size[this->dims - 1]);