avcodec/jpeg2000dec: Fix division by zero in jp2_find_codestream()

Fixes: 2707/clusterfuzz-testcase-minimized-5179636394754048

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