[3.0] 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.

(cherry picked from commit 321a29b0)