Fix CVE-2021-3498:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-3498
https://gstreamer.freedesktop.org/security/sa-2021-0003.html

Patch copied from upstream source repository:

https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/commit/02174790726dd20a5c73ce2002189bf240ad4fe0?merge_request_iid=903

diff --git a/gst/matroska/matroska-demux.c b/gst/matroska/matroska-demux.c
index 4d0234743b8cf243b4521e56ef9027ba23b1b5d0..467815986c8c3d86fd8906a0d539b34f67d6693e 100644
--- a/gst/matroska/matroska-demux.c
+++ b/gst/matroska/matroska-demux.c
@@ -692,6 +692,8 @@ gst_matroska_demux_parse_stream (GstMatroskaDemux * demux, GstEbmlRead * ebml,
 
   DEBUG_ELEMENT_START (demux, ebml, "TrackEntry");
 
+  *dest_context = NULL;
+
   /* start with the master */
   if ((ret = gst_ebml_read_master (ebml, &id)) != GST_FLOW_OK) {
     DEBUG_ELEMENT_STOP (demux, ebml, "TrackEntry", ret);
on>
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<tr><td><span title='2022-04-29 18:07:16 +0200'>2022-04-29</span></td><td><a href='/guix/commit/gnu/services/sysctl.scm?id=9d7248cd87fbda8c717341a8092780782bef0820'>services: Add missing 'description' fields.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-03-31 15:36:49 +0200'>2021-03-31</span></td><td><a href='/guix/commit/gnu/services/sysctl.scm?id=4f547c50f9bc478fd7c8bb9d481e2e376b969e8a'>services: sysctl: Export &lt;sysctl-configuration&gt; record field accessors.</a><span class='msg-avail'>...</span></td><td>muradm</td></tr>
<tr><td><span title='2021-03-18 12:17:33 +0100'>2021-03-18</span></td><td><a href='/guix/commit/gnu/services/sysctl.scm?id=898489f48e436e45e86e1ba0fcdb6df5cd5a051a'>services: Enable "protected hardlinks" and "protected symlinks" by default.</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>