avcodec/cllc: Check prefix

Fixes: runtime error: left shift of 1610706944 by 1 places cannot be represented in type 'int'
Fixes: 1421/clusterfuzz-testcase-minimized-6239947507892224

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
