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>
