AE: Workaround (ugly) non existing channel maps in old ffmpeg #14407

Old ffmpeg lacks a complete channel_map. So we can end up empty and run into an assert later. This patch workarounds it by choosing the next lower available profile.

I don't call it fix, cause a proper fix would incorporate the user layout, e.g. 6.1 -> 5.1 but 7.0 gets 5.0 with proper channel mixing.

ffmpeg 1.2 (master) has a full channel_map, so no problem here.