avcodec/utils: Require a non zero channels unless AV_CODEC_CAP_CHANNEL_CONF is set

avcodec/utils: Require a non zero channels unless AV_CODEC_CAP_CHANNEL_CONF is set

This avoids per codec checks for channels not being 0

Fixes: division by 0
Fixes: 25419/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FASTAUDIO_fuzzer-5632544761184256
Fixes: 25433/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FASTAUDIO_fuzzer-6215671900536832
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Suggested-by: Paul B Mahol <onemda@gmail.com>
See: [FFmpeg-devel] [PATCH 1/3] avcodec/fastaudio: Check channel
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>