avcodec/utvideodec: Set pro flag based on fourcc

      This avoids mixing 8bit variants with pro and 10bit with non pro mode.
Fixes: out of array read
Fixes: poc_03_30.avi

Found-by: GwanYeong Kim <gy741.kim@gmail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>