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 #92 (closed)