Fix T54225: Blur node stopped working when Map Range was fed with image

The issue was happening with fast Gaussian blur, and caused by NaN value pixels
in the input buffer.

Now made it so Map Range output does not produce NaN, by returning arbitrary
value of 0. Still better than NaN!

source/blender/compositor/operations/COM_MapRangeOperation.cpp

@@ -65,6 +65,11 @@ void MapRangeOperation::executePixelSampled(float output[4], float x, float y, P
 	dest_min = inputs[3];
 	dest_max = inputs[4];
 
+	if (fabsf(source_max - source_min) < 1e-6f) {
+		output[0] = 0.0f;
+		return;
+	}
+
 	if (value >= -BLENDER_ZMAX && value <= BLENDER_ZMAX) {
 		value = (value - source_min) / (source_max - source_min);
 		value = dest_min + value * (dest_max - dest_min);