Code should make more sense now

Fixes: out of array access
Fixes: 58299/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_JPEG2000_fuzzer-6627570448465920

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