avcodec/mpeg_er: Clear mcsel in mpeg_er_decode_mb()

Fixes out of array read
Should fix: 3516/clusterfuzz-testcase-minimized-4608518562775040 (not reprodoceable)

Found-by: Insu Yun, Georgia Tech.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 127a362630)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpeg_er.c

@@ -71,6 +71,7 @@ static void mpeg_er_decode_mb(void *opaque, int ref, int mv_dir, int mv_type,
     s->mb_skipped = mb_skipped;
     s->mb_x       = mb_x;
     s->mb_y       = mb_y;
+    s->mcsel      = 0;
     memcpy(s->mv, mv, sizeof(*mv));
 
     ff_init_block_index(s);