fixed SIFT when keypoints vector is empty (#1162)

modules/features2d/src/sift.cpp

@@ -1722,6 +1722,7 @@ void SIFT::operator()(const Mat& image, const Mat& mask,
         KeyPointsFilter::removeDuplicated( keypoints );
 
         // Compute descriptors.
+        if( featuresSeq->total > 0 )
             cvSeqRemoveSlice( featuresSeq, cvSlice(0, featuresSeq->total) );
     }