avcodec/jpeg2000: Dynamically allocate codeblock data

Fixes: OOM
Fixes: 3541/clusterfuzz-testcase-minimized-6469958596820992

Adds support for decoding codeblock data larger than 8kb
Reduces decoder memory consumption

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