avcodec/atrac9dec: Check q_unit_cnt more completely before using it to access at9_tab_band_ext_group

Fixes: index 8 out of bounds for type 'const uint8_t [8][3]'
Fixes: 19127/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ATRAC9_fuzzer-5709394985091072

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