added epsilon value to weights in the MergeMertens in order to avoid zero…

added epsilon value to weights in the MergeMertens in order to avoid zero weights for pixels from uniformly filled areas of image

modules/photo/src/merge.cpp

@@ -208,7 +208,7 @@ public:
             if(channels == 3) {
                 weights[i] = weights[i].mul(saturation);
             }
-            weights[i] = weights[i].mul(wellexp);
+            weights[i] = weights[i].mul(wellexp) + 1e-12f;
             weight_sum += weights[i];
         }
         int maxlevel = static_cast<int>(logf(static_cast<float>(min(size.width, size.height))) / logf(2.0f));