oggdec: prevent heap corruption.

Specifically crafted samples can reinit ogg->streams[] while
reading samples, and thus we should not cache old pointers since
these may no longer be valid.
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavformat/oggdec.c

@@ -592,15 +592,15 @@ static int64_t ogg_read_timestamp(AVFormatContext *s, int stream_index,
                                   int64_t *pos_arg, int64_t pos_limit)
 {
     struct ogg *ogg = s->priv_data;
-    struct ogg_stream *os = ogg->streams + stream_index;
     AVIOContext *bc = s->pb;
     int64_t pts = AV_NOPTS_VALUE;
-    int i;
+    int i = -1;
     avio_seek(bc, *pos_arg, SEEK_SET);
     ogg_reset(ogg);
 
     while (avio_tell(bc) < pos_limit && !ogg_packet(s, &i, NULL, NULL, pos_arg)) {
         if (i == stream_index) {
+            struct ogg_stream *os = ogg->streams + stream_index;
             pts = ogg_calc_pts(s, i, NULL);
             if (os->keyframe_seek && !(os->pflags & AV_PKT_FLAG_KEY))
                 pts = AV_NOPTS_VALUE;
@@ -626,6 +626,7 @@ static int ogg_read_seek(AVFormatContext *s, int stream_index,
         os->keyframe_seek = 1;
 
     ret = av_seek_frame_binary(s, stream_index, timestamp, flags);
+    os = ogg->streams + stream_index;
     if (ret < 0)
         os->keyframe_seek = 0;
     return ret;