avcodec/cfhd: Fix undefined shift

Fixes: runtime error: left shift of negative value -1
Fixes: 2303/clusterfuzz-testcase-minimized-5529675273076736

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