avcodec: ensure valid buffer size for avcodec_fill_audio_frame

avcodec_fill_audio_frame will reject input buffers that are too small,
therefore we cannot call it directly with the input buffer p_aout_buf.

Signed-off-by: Ilkka Ollakka <ileoo@videolan.org>
avcodec: reuse DEFAULT_ALIGN

Signed-off-by: Ilkka Ollakka <ileoo@videolan.org>