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);
alue='switch'/></form></td></tr>
<tr><td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
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<div class='path'>path: <a href='/guix/log/?id=13e889a7583a38669f6c5feacea872945a3a1522&amp;showmsg=1'>root</a>/<a href='/guix/log/scripts?id=13e889a7583a38669f6c5feacea872945a3a1522&amp;showmsg=1'>scripts</a>/<a href='/guix/log/scripts/guix.in?id=13e889a7583a38669f6c5feacea872945a3a1522&amp;showmsg=1'>guix.in</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/scripts/guix.in?id=13e889a7583a38669f6c5feacea872945a3a1522'>Collapse</a>)</th><th class='left'>Author</th></tr>
<tr class='logheader'><td><span title='2018-06-09 12:02:27 +0200'>2018-06-09</span></td><td class='logsubject'><a href='/guix/commit/scripts/guix.in?id=75e24d7b0e19ab2164aecb340e82d07d2b9714e7'>pull: Install the new Guix in a profile.</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* guix/scripts/pull.scm (%pull-version): New variable.
(build-from-source): Pass #:pull-version to BUILD.
(whole-package-for-legacy, derivation-&gt;manifest-entry): New procedure.
(build-and-install): Rewrite in terms of 'build-and-use-profile'.
* guix/scripts/system.scm (maybe-suggest-running-guix-pull)[latest]:
Switch to "/current".
* scripts/guix.in (augment-load-paths!): Remove use of
~/.config/guix/latest.
* build-aux/compile-as-derivation.scm: Replace "/guix/latest/" with
"/current/share/guile/site/X.Y"
* guix/scripts.scm (warn-about-old-distro)[age]: Check "/current"
instead of "/latest".
* doc/guix.texi (Invoking guix pull): Document it.
* doc/contributing.texi (Running Guix Before It Is Installed): Remove
footnote about abusing ~/.config/guix/latest.


</td></tr>
<tr class='logheader'><td><span title='2018-01-28 23:14:55 +0100'>2018-01-28</span></td><td class='logsubject'><a href='/guix/commit/scripts/guix.in?id=63666a348af5ef5190cbe9611c40f2b9188b7967'>guix: Let Emacs detect “scripts/guix.in” appropriate mode.</a></td><td>Mathieu Lirzin</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
Since commit 6f774d481839f87178c5895ac2d661e141f879b8 which introduces the use
of Guile's meta switch in “scripts/guix.in”, Emacs was not using ‘scheme-mode’
for this file.

* scripts/guix.in: Replace "-*- scheme -*-" with a local variable.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;


</td></tr>
<tr class='logheader'><td><span title='2018-01-23 15:01:15 +0100'>2018-01-23</span></td><td class='logsubject'><a href='/guix/commit/scripts/guix.in?id=6f774d481839f87178c5895ac2d661e141f879b8'>guix: Refactor script.</a></td><td>Mathieu Lirzin</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* scripts/guix.in: Remove empty surrounding ‘let’.  Define 'main' as the
procedure called when running the script.
(maybe-augment-load-paths!): Rename to ...
(augment-load-paths!): ... this.  Use 'and=&gt;' for 'file-exists?'.
(run-guix-main): Rename to ...
(main): ... this.  Call 'augment-load-paths!'.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;


</td></tr>
<tr class='logheader'><td><span title='2018-01-23 10:13:16 +0100'>2018-01-23</span></td><td class='logsubject'><a href='/guix/commit/scripts/guix.in?id=cba386c1295b7054df22dfb4662b1b92f668e2c0'>build: Expand ‘scripts/guix’ at Make time.</a></td><td>Mathieu Lirzin</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
This moves the complexity of Autotools variable expansion outside of the
application code.

* scripts/guix.in (config-lookup): Delete.
(maybe-augment-load-paths!, run-guix-main): Use fully expanded variables
instead of calling ‘config-lookup’.
* configure.ac: Don't use AC_CONFIG_FILES for ‘scripts/guix’.
Use AC_PROG_SED.
* Makefile.am (scripts/guix): New rule.
(do_subst): New variable.
(CLEANFILES, EXTRA_DIST): Adapt.

Co-authored-by: Ludovic Courtès &lt;ludo@gnu.org&gt;


</td></tr>
<tr class='logheader'><td><span title='2017-03-20 23:26:18 +0100'>2017-03-20</span></td><td class='logsubject'><a href='/guix/commit/scripts/guix.in?id=9437fd7399425ed281e1bfc13240288ba9078caa'>build: Install .go files to $libdir/guile/X.Y.</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* configure.ac: Define and substitute 'guileobjectdir'.
* Makefile.am (nobase_nodist_guilemodule_DATA): Remove $(GOBJECTS).
(nobase_nodist_guileobject_DATA): New variable.
(guix_install_go_files): Adjust accordingly.
(install-data-hook): Likewise.
* scripts/guix.in (config-lookup): Add 'exec_prefix' and
'guileobjectdir'.  Add '_' in VAR-REF-REGEXP.
(maybe-augment-load-paths!): Distinguish OBJECT-DIR from MODULE-DIR.


</td></tr>
<tr class='logheader'><td><span title='2016-04-06 23:21:26 +0200'>2016-04-06</span></td><td class='logsubject'><a href='/guix/commit/scripts/guix.in?id=7cffaeb60473e25a7ef8c1a8cb36e1531191873a'>challenge: Really exit with non-zero upon hash mismatch.</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
Reported by John Darrington.

* guix/scripts/challenge.scm (guix-challenge): Add an explicit 'exit'
call when ISSUES is empty.
* scripts/guix.in: Add comment about 'exit'.
* doc/guix.texi (Invoking guix challenge): Mention the behavior and exit
code.


</td></tr>