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;
     }
   }
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