mremap_chunk(): invalid pointer

I don't know if it's an issue with these bindings, or the original library, or if I'm doing something terrible...
My application crashed in a very rare case when I was in the folder selector opened with `nfd::open_pick_folder`.
And gave output:

```
mremap_chunk(): invalid pointer
munmap_chunk(): invalid pointer
```
The code: https://github.com/Ploppz/bup/blob/main/src/path.rs