ff_parse_specific_params: do not use AVCodecContext.frame_size

It will not be set unless the muxing codec context is also the encoding
context, which is discouraged. When the frame size is not known from
av_get_audio_frame_duration(), the fallback should still be good enough.

libavformat/riffenc.c

@@ -212,12 +212,7 @@ void ff_parse_specific_params(AVStream *st, int *au_rate,
     int gcd;
     int audio_frame_size;
 
-    /* We use the known constant frame size for the codec if known, otherwise
-     * fall back on using AVCodecContext.frame_size, which is not as reliable
-     * for indicating packet duration. */
     audio_frame_size = av_get_audio_frame_duration(codec, 0);
-    if (!audio_frame_size)
-        audio_frame_size = codec->frame_size;
 
     *au_ssize = codec->block_align;
     if (audio_frame_size && codec->sample_rate) {