Fixes: out of array read
Fixes: 19309/clusterfuzz-testcase-minimized-ffmpeg_BSF_MP3_HEADER_DECOMPRESS_fuzzer-5651002950942720

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