WavPack demuxer: store position of the first block in index.

Currently for multichannel audio position for the last block position is
stored in index (and used for seeking), which is obviously not correct.
Signed-off-by: NLuca Barbato <lu_zero@gentoo.org>

libavformat/wv.c

@@ -250,6 +250,7 @@ static int wv_read_packet(AVFormatContext *s,
     WVContext *wc = s->priv_data;
     int ret;
     int size, ver, off;
+    int64_t pos;
 
     if (s->pb->eof_reached)
         return AVERROR(EIO);
@@ -258,6 +259,7 @@ static int wv_read_packet(AVFormatContext *s,
             return -1;
     }
 
+    pos = wc->pos;
     off = wc->multichannel ? 4 : 0;
     if(av_new_packet(pkt, wc->blksize + WV_EXTRA_SIZE + off) < 0)
         return AVERROR(ENOMEM);
@@ -314,7 +316,7 @@ static int wv_read_packet(AVFormatContext *s,
     pkt->stream_index = 0;
     wc->block_parsed = 1;
     pkt->pts = wc->soff;
-    av_add_index_entry(s->streams[0], wc->pos, pkt->pts, 0, 0, AVINDEX_KEYFRAME);
+    av_add_index_entry(s->streams[0], pos, pkt->pts, 0, 0, AVINDEX_KEYFRAME);
     return 0;
 }