dual demosaic auto contrast: fix segfault when falling back to zero, #4866

rtengine/rt_algo.cc

@@ -286,6 +286,13 @@ void buildBlendMask(float** luminance, float **blend, int W, int H, float &contr
             }
         }
 
+        if(contrastThreshold == 0.f) {
+            for(int j = 0; j < H; ++j) {
+                for(int i = 0; i < W; ++i) {
+                    blend[j][i] = amount;
+                }
+            }
+        } else {
 #ifdef _OPENMP
             #pragma omp parallel
 #endif
@@ -346,6 +353,7 @@ void buildBlendMask(float** luminance, float **blend, int W, int H, float &contr
                 gaussianBlur(blend, blend, W, H, 2.0);
             }
         }
+    }
 }
 
 int calcContrastThreshold(float** luminance, float **blend, int W, int H) {