Fix a stack corruption when handling files with more than 64 audio
channels:

https://gstreamer.freedesktop.org/security/sa-2021-0005.html

Patch copied from upstream source repository:

https://gitlab.freedesktop.org/gstreamer/gst-libav/-/commit/dcea8baa14a5fc3b796d876baaf2f238546ba2b1

diff --git a/ext/libav/gstavcodecmap.c b/ext/libav/gstavcodecmap.c
index b5be4bb7a5f2712f78383da9319754a8849e3307..be22f22cf5c7c7b22b13e44b10999adaacbcca2b 100644
--- a/ext/libav/gstavcodecmap.c
+++ b/ext/libav/gstavcodecmap.c
@@ -102,7 +102,7 @@ gst_ffmpeg_channel_layout_to_gst (guint64 channel_layout, gint channels,
   guint nchannels = 0;
   gboolean none_layout = FALSE;
 
-  if (channel_layout == 0) {
+  if (channel_layout == 0 || channels > 64) {
     nchannels = channels;
     none_layout = TRUE;
   } else {
@@ -163,7 +163,7 @@ gst_ffmpeg_channel_layout_to_gst (guint64 channel_layout, gint channels,
     } else {
       guint i;
 
-      for (i = 0; i < nchannels; i++)
+      for (i = 0; i < nchannels && i < 64; i++)
         pos[i] = GST_AUDIO_CHANNEL_POSITION_NONE;
     }
   }
href='/guix/refs/?id=7bd5394bf8564920a4e647563dc8c0092858f8dd'>refs</a><a class='active' href='/guix/log/etc/git?showmsg=1'>log</a><a href='/guix/tree/etc/git?id=7bd5394bf8564920a4e647563dc8c0092858f8dd'>tree</a><a href='/guix/commit/etc/git?id=7bd5394bf8564920a4e647563dc8c0092858f8dd'>commit</a><a href='/guix/diff/etc/git?id=7bd5394bf8564920a4e647563dc8c0092858f8dd'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/etc/git'>
<input type='hidden' name='id' value='7bd5394bf8564920a4e647563dc8c0092858f8dd'/><input type='hidden' name='showmsg' value='1'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=7bd5394bf8564920a4e647563dc8c0092858f8dd&amp;showmsg=1'>root</a>/<a href='/guix/log/etc?id=7bd5394bf8564920a4e647563dc8c0092858f8dd&amp;showmsg=1'>etc</a>/<a href='/guix/log/etc/git?id=7bd5394bf8564920a4e647563dc8c0092858f8dd&amp;showmsg=1'>git</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/etc/git?id=7bd5394bf8564920a4e647563dc8c0092858f8dd'>Collapse</a>)</th><th class='left'>Author</th></tr>
<tr class='logheader'><td><span title='2021-10-14 00:24:34 +0200'>2021-10-14</span></td><td class='logsubject'><a href='/guix/commit/etc/git?id=f8b10932c86cf4c1657a67e8c484d76f757b64bd'>Add git configuration templates to improve diff hunk header detection.</a></td><td>Sarah Morgensen</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
This is a follow-up to commit 9fc8ae4171e5da4939a64fc6d684c8b9d85bbe84, which
missed two hunks from &lt;https://issues.guix.gnu.org/50363&gt;.

Reported by hackeryarn on #guix.

* .gitattributes, etc/git/gitconfig: New files.

Signed-off-by: Marius Bakke &lt;marius@gnu.org&gt;


</td></tr>
<tr class='logheader'><td><span title='2020-12-14 12:15:06 -0500'>2020-12-14</span></td><td class='logsubject'><a href='/guix/commit/etc/git?id=80ebcdd100a82fdc582e62f35042c74ce38ea753'>maint: Only run `make authenticate` when pushing commits.</a></td><td>Leo Famulari</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* etc/git/pre-push: Exit early when deleting a branch.


</td></tr>
<tr class='logheader'><td><span title='2020-06-01 00:51:38 +0200'>2020-06-01</span></td><td class='logsubject'><a href='/guix/commit/etc/git?id=6f6758c45d3913dbf98bb8e50f21f7bfb25b40ea'>maint: Adjust comment in 'pre-push' hook.</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* etc/git/pre-push: Adjust comment.


</td></tr>
<tr class='logheader'><td><span title='2020-05-29 18:31:38 +0200'>2020-05-29</span></td><td class='logsubject'><a href='/guix/commit/etc/git?id=e65a44649e8d7698c4a888f1de625a67052520e9'>maint: Git pre-push hook runs "make authenticate check-channel-news".</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* etc/git/pre-push: Change to run "make authenticate check-channel-news".


</td></tr>