avcodecmap: Don't try converting channel layouts with more than 64 channels

We only support up to 64 channels in GStreamer with a specific layout so
it's safe to assume a NONE layout in this case.

Also the arrays of channel positions are allocated everywhere with 64
elements so don't try setting more than 64 to NONE as that will cause
stack corruptions and similar memory safety issues.

Thanks to Natalie Silvanovich for reporting this issue.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-libav/-/issues/92

Part-of: <gstreamer/gst-libav!121>
30 jobs for !121 with backport-mr-120-into-1.18-channel-layout-64-plus-channels in 33 minutes and 47 seconds (queued for 4 seconds)
detached
Status Job ID Name Coverage
  Build Docker
passed #8003098
alpine amd64 manifest builder docker

00:00:17

passed #8003097
android docker

00:00:18

passed #8003096
cerbero fedora amd64 docker

00:00:24

passed #8003095
fedora amd64 docker

00:00:23

passed #8003099
gst-indent amd64 docker

00:00:23

 
  Preparation
passed #8003101
gst indent

00:00:08

passed #8003100
manifest

00:00:16

 
  Build
passed #8003110
gstreamer
build cerbero cross-android universal

00:11:29

passed #8003114
gst-ios-14.3
build cerbero cross-ios universal

00:20:28

passed #8003111
gstreamer
build cerbero cross win32

00:08:18

passed #8003112
gstreamer
build cerbero cross win64

00:07:41

passed #8003109
gstreamer
build cerbero fedora x86_64

00:07:40

passed #8003113
gst-macos-11.1
build cerbero macos x86_64

00:11:50

passed #8003105
build clang fedora x86_64

00:08:45

manual #8003108
docker windows 1809 allowed to fail manual
build msys2
passed #8003102
build nodebug fedora x86_64

00:08:01

passed #8003103
build static fedora x86_64

00:08:06

passed #8003104
build static nodebug fedora x86_64

00:08:00

passed #8003106
docker windows 1809
build vs2017 amd64

00:08:57

passed #8003107
docker windows 1809
build vs2017 x86

00:09:09

 
  Test
passed #8003115
gstreamer
check fedora

00:07:48

passed #8003116
gstreamer
integration testsuites fedora 1/4

00:15:08

passed #8003117
gstreamer
integration testsuites fedora 2/4

00:15:35

passed #8003118
gstreamer
integration testsuites fedora 3/4

00:14:13

passed #8003119
gstreamer
integration testsuites fedora 4/4

00:19:17

 
  Integrate
passed #8003857
cross-android universal examples

00:08:09

passed #8003122
gst-ios-14.3
cross-ios universal examples

00:04:27

passed #8003121
documentation

00:14:21

failed #8003120
cross-android universal examples

00:05:07

failed #8003466
cross-android universal examples

00:05:54