avcodec/me_cmp: Fix median_sad size

Fixes out of array read
Fixes: COV1396255
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/me_cmp.h

@@ -76,7 +76,7 @@ typedef struct MECmpContext {
     me_cmp_func frame_skip_cmp[6]; // only width 8 used
 
     me_cmp_func pix_abs[2][4];
-    me_cmp_func median_sad[2];
+    me_cmp_func median_sad[6];
 } MECmpContext;
 
 void ff_me_cmp_init_static(void);