avcodec/motion_est: Fix xy indexing on range violation in ff_get_best_fcode()

This codepath seems untested, no testcases change

Found-by: <mkver>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/motion_est.c

@@ -1613,7 +1613,7 @@ int ff_get_best_fcode(MpegEncContext * s, int16_t (*mv_table)[2], int type)
         for(y=0; y<s->mb_height; y++){
             int x;
             int xy= y*s->mb_stride;
-            for(x=0; x<s->mb_width; x++){
+            for(x=0; x<s->mb_width; x++, xy++){
                 if(s->mb_type[xy] & type){
                     int mx= mv_table[xy][0];
                     int my= mv_table[xy][1];
@@ -1630,7 +1630,6 @@ int ff_get_best_fcode(MpegEncContext * s, int16_t (*mv_table)[2], int type)
                             score[j]-= 170;
                     }
                 }
-                xy++;
             }
         }