Fixes: use after free
Fixes: 22484/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_JPEG2000_fuzzer-5671488765296640

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