libvorbisdec: dont try to update an empty que

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/libvorbisenc.c

@@ -358,7 +358,7 @@ static int oggvorbis_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
     if (duration > 0) {
         /* we do not know encoder delay until we get the first packet from
          * libvorbis, so we have to update the AudioFrameQueue counts */
-        if (!avctx->delay) {
+        if (!avctx->delay && s->afq.frames) {
             avctx->delay              = duration;
             av_assert0(!s->afq.remaining_delay);
             s->afq.frames->duration  += duration;