avcodec/ac3dec: Keep track of band structure

It is needed in some corner cases that seem not to be forbidden
Fixes: out of array index
Fixes: 1538/clusterfuzz-testcase-minimized-4696904925446144

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