fixed the error in SURF when no keypoint detected

modules/features2d/src/surf.cpp

@@ -662,7 +662,6 @@ const float SURFInvoker::DESC_SIGMA = 3.3f;
     
 }
 
-
 CV_IMPL void
 cvExtractSURF( const CvArr* _img, const CvArr* _mask,
                CvSeq** _keypoints, CvSeq** _descriptors,
@@ -762,6 +761,7 @@ cvExtractSURF( const CvArr* _img, const CvArr* _mask,
     }
     cvScale( &_DW, &_DW, 1./gs );
 	
+	if ( N > 0 )
 		cv::parallel_for(cv::BlockedRange(0, N),
 						 cv::SURFInvoker(&params, keypoints, descriptors, img, sum,
 										 apt, aptw, nangle0, &DW[0][0]));