Fixes: passing zero to clz(), which is not a valid argument
Fixes: 23337/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PIXLET_fuzzer-5179131989065728

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit bd0f81526d3f4c23ecd0a399829103be2445c011)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>