matroskadec: Fix a bug where a pointer was cached to an array that might later move due to a realloc()

Fixes bug #190
Chromium bug #100492
related to CVE-2011-3893

Signed-off-by: Reinhard Tartler <siretart@tauware.de>

libavformat/matroskadec.c

@@ -1188,7 +1188,6 @@ static int matroska_parse_seekhead_entry(MatroskaDemuxContext *matroska, int idx
 static void matroska_execute_seekhead(MatroskaDemuxContext *matroska)
 {
     EbmlList *seekhead_list = &matroska->seekhead;
-    MatroskaSeekhead *seekhead = seekhead_list->elem;
     int64_t before_pos = avio_tell(matroska->ctx->pb);
     int i;
 
@@ -1198,6 +1197,7 @@ static void matroska_execute_seekhead(MatroskaDemuxContext *matroska)
         return;
 
     for (i = 0; i < seekhead_list->nb_elem; i++) {
+        MatroskaSeekhead *seekhead = seekhead_list->elem;
         if (seekhead[i].pos <= before_pos)
             continue;