avcodec/ylc: Fix vlc of 31 bits

Fixes: runtime error: left shift of 1 by 31 places cannot be represented in type 'int'
Fixes: 2515/clusterfuzz-testcase-minimized-6197200012967936

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
