Problem:
The fold refresh path for foldminlines/foldnestmax creates a new
FoldInfo and starts an async parse. If FileType or BufUnload re-enters
before that callback returns, foldinfos[bufnr] can be cleared or
replaced. The callback then indexes a stale slot and raises an "attempt
to index a nil value" error.
Solution:
Capture the FoldInfo created for the refresh and carry that object
through the async callback. Before calling foldupdate(), verify that the
buffer still points at the same FoldInfo generation; otherwise ignore
the stale callback.
AI-assisted: Codex
Fixes#38461
ea878f456a