swfenc: use av_get_audio_frame_duration() instead of AVCodecContext.frame_size

This way we can do stream copy without having the demuxer wait until
frame_size has been set.

libavformat/swfenc.c

@@ -187,10 +187,6 @@ static int swf_write_header(AVFormatContext *s)
         AVCodecContext *enc = s->streams[i]->codec;
         if (enc->codec_type == AVMEDIA_TYPE_AUDIO) {
             if (enc->codec_id == CODEC_ID_MP3) {
-                if (!enc->frame_size) {
-                    av_log(s, AV_LOG_ERROR, "audio frame size not set\n");
-                    return -1;
-                }
                 swf->audio_enc = enc;
                 swf->audio_fifo= av_fifo_alloc(AUDIO_FIFO_SIZE);
                 if (!swf->audio_fifo)
@@ -452,7 +448,7 @@ static int swf_write_audio(AVFormatContext *s,
     }
 
     av_fifo_generic_write(swf->audio_fifo, buf, size, NULL);
-    swf->sound_samples += enc->frame_size;
+    swf->sound_samples += av_get_audio_frame_duration(enc, size);
 
     /* if audio only stream make sure we add swf frames */
     if (!swf->video_enc)