Set frame_size to the gcd of what the decoder can output.

Originally committed as revision 13971 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/vorbis_dec.c

@@ -979,6 +979,7 @@ static av_cold int vorbis_decode_init(AVCodecContext *avccontext) {
 
     avccontext->channels = vc->audio_channels;
     avccontext->sample_rate = vc->audio_samplerate;
+    avccontext->frame_size  = FFMIN(vc->blocksize[0], vc->blocksize[1])>>2;
 
     return 0 ;
 }