Fixes: left shift of negative value -1
Fixes: 57554/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FFVHUFF_fuzzer-4853603839115264

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