The issue was that the forward compatibility saving code was setting the
group's `channels` listbase to point at the forward-compatible fcurve
listbase, but the `channels` listbase wasn't later getting cleared when
loading the groups as part of a layered action. In fact, the listbase
pointers were being completely ignored in that case because they aren't
relevant to groups on layered actions, and therefore weren't getting
remapped to the new correct memory addresses of the fcurves.
The end result was that after loading, the group's `channels` listbase
would be non-null and pointing to invalid memory, and there are some
code paths that then try to use that listbase, resulting in a segfault.
This commit fixes the issue by ensuring that the groups' `channels`
listbases are cleared when loading them as part of a layered action.
Additionally, we now also clear them after they're set during
forward-compatibly saving, the lack of which wasn't immediately
causing problems but was nevertheless incorrect.
Pull Request: https://projects.blender.org/blender/blender/pulls/126834
e3311e48c0