Fixed crash in SSE implementation of FREAK descriptor when number of pairs is…

Fixed crash in SSE implementation of FREAK descriptor when number of pairs is set to something other than 512.

See http://code.opencv.org/issues/3889 for more details

modules/features2d/src/freak.cpp

@@ -394,7 +394,7 @@ void FREAK::computeImpl( const Mat& image, std::vector<KeyPoint>& keypoints, Mat
                 (*ptr) = result128;
                 ++ptr;
             }
-            ptr -= 8;
+            ptr -= (FREAK_NB_PAIRS/128)*2;
 #else
             // extracting descriptor preserving the order of SSE version
             int cnt = 0;