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 so
that |tracks| can get invalidated.

This CL assigns |tracks| only before we use it so that it won't be invalidated.

BUG=427266
TEST=Test case in associated bug passes now.

Change-Id: I9c7065fe8f4311ca846076281df2282d190ed344
diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index 5abf0f8..42dbd81 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -2925,7 +2925,7 @@
                               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;
 
@@ -2955,6 +2955,7 @@
         goto err;
 
     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;