avcodec: fill audio frame channel field
Since ffmpeg 4.4 specifying channel layout or channel number is required
to call avcodec_send_frame()
without error.
This is new requirement is due to a change in the avcodec internal API
brought by 827d6fe73d2f5472c1c2128eb14fab6a4db29032 (ffmpeg repo). They
now call av_frame_ref
which needs the channels fields to be valid.
Fixes: #25851 (closed)