Made small change which fixes the situation, when ORB does not find any…

Made small change which fixes the situation, when ORB does not find any descriptors on an image. Earlier ORB::operator() crashed in this case, since rowRange method was called for an empty matrix.

modules/features2d/src/orb.cpp

@@ -932,7 +932,12 @@ void ORB::operator()( InputArray _image, InputArray _mask, vector<KeyPoint>& _ke
         // Compute the descriptors
         if (do_descriptors)
         {
-            Mat desc = descriptors.rowRange(offset, offset + nkeypoints);
+            Mat desc;
+            if (!descriptors.empty()) 
+            {
+                desc = descriptors.rowRange(offset, offset + nkeypoints);
+            } 
+
             offset += nkeypoints;
             // preprocess the resized image
             Mat& workingMat = imagePyramid[level];