avcodec/cfhd: Keep track of which subbands have been read

avcodec/cfhd: Keep track of which subbands have been read

This avoids use of uninitialized data
also several checks are inside the band reading code
so it is important that it is run at least once

Fixes: out of array accesses
Fixes: 28209/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_CFHD_fuzzer-5684714694377472
Fixes: 32124/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_CFHD_fuzzer-5425980681355264
Fixes: 30519/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_CFHD_fuzzer-4558757155700736
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>