matroskadec: Fix read-after-free in matroska_read_seek()

In matroska_read_seek(), |tracks| is assigned at the begining of the
function. However, functions like matroska_parse_cues() could reallocate
the tracks and invalidate |tracks|.

This assigns |tracks| only before using it, so that it will not get
invalidated elsewhere.

Bug-Id: chromium/427266

libavformat/matroskadec.c

@@ -2541,7 +2541,7 @@ static int matroska_read_seek(AVFormatContext *s, int stream_index,
                               int64_t timestamp, int flags)
 {
     MatroskaDemuxContext *matroska = s->priv_data;
-    MatroskaTrack *tracks = matroska->tracks.elem;
+    MatroskaTrack *tracks = NULL;
     AVStream *st = s->streams[stream_index];
     int i, index, index_sub, index_min;
 
@@ -2571,6 +2571,7 @@ static int matroska_read_seek(AVFormatContext *s, int stream_index,
         return 0;
 
     index_min = index;
+    tracks = matroska->tracks.elem;
     for (i = 0; i < matroska->tracks.nb_elem; i++) {
         tracks[i].audio.pkt_cnt        = 0;
         tracks[i].audio.sub_packet_cnt = 0;