AVFormat Muxer : Set codec frame size.

This allows for correct output when using muxers (like the MOV/MP4 muxer)
that require frame_size set.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit 077f47ff59b9e5e1a523d924a0307105dcdee4bc)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/avformat/mux.c

@@ -232,6 +232,7 @@ static int AddStream( sout_mux_t *p_mux, sout_input_t *p_input )
         codec->channels = p_input->p_fmt->audio.i_channels;
         codec->sample_rate = p_input->p_fmt->audio.i_rate;
         codec->time_base = (AVRational){1, codec->sample_rate};
+        codec->frame_size = p_input->p_fmt->audio.i_frame_length;
         break;
 
     case VIDEO_ES: