Do not use = 0 for a cv::Mat.

There are several operator= overloads and some compilers can be confused.
This is a leftover from #2987

Author: vrabaud

modules/img_hash/src/radial_variance_hash.cpp

@@ -266,7 +266,7 @@ class RadialVarianceHashImpl CV_FINAL : public ImgHashBase::ImgHashImpl
         }
         else
         {
-            hash = 0;
+            hash.setTo(cv::Scalar::all(0));
         }
     }