avcodec/jpeg2000dec: Fixes invalid shifts in jpeg2000_decode_packets_po_iteration()

Fixes: shift exponent 47 is too large for 32-bit type 'int'
Fixes: 9163/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_JPEG2000_fuzzer-5661750182543360

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>