libtta 2.3 has a limit of 6 channels, so 16 is substantially above the "official" already

Fixes: OOM
Fixes: 15249/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TTA_fuzzer-5643988125614080

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