ogg: in ogg_get_length() start from data_offset not 0.

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

libavformat/oggdec.c

@@ -520,7 +520,7 @@ static int ogg_get_length(AVFormatContext *s)
     ogg_restore (s, 0);
 
     ogg_save (s);
-    avio_seek (s->pb, 0, SEEK_SET);
+    avio_seek (s->pb, s->data_offset, SEEK_SET);
     while (!ogg_read_page (s, &i)){
         if (ogg->streams[i].granule != -1 && ogg->streams[i].granule != 0 &&
             ogg->streams[i].codec) {