Fix autoreload crash / error

[vqns]

the autoreload plugin may cause errors or crashes when a doc with a (absolute) filename is no longer backed by an actual on disk file.

steps to reproduce:

1. have lite-xl with a file open in a doc
2. delete the file, keep the doc open
3. create a folder with the same name as the file

causes an error if done from within lite-xl (e.g the treeview), a crash if done externally

The deeper issue is that docs keep their filename even when their backing files are deleted, however as far as i can make sense of the dirwatch, there doesn't seem to be a way to tell apart a rename from a deletion + creation. So clearing filenames would also make externally renamed docs dangling (which may be desired behavior?)

See:
https://github.com/lite-xl/lite-xl/blob/master/data/core/init.lua#L199

[takase1121]

I wasn't able to replicate this on Windows.

[FijiHasGithub]

Can reproduce both the error and the crash on Alpine Linux 6.6.12, using Lite-XL built from the master branch

[Guldoman]

Can reproduce on Arch too.
I don't know if this should be fixed here or in the backend.
@adamharrison might have some ideas.

[vqns]

I don't know if this should be fixed here or in the backend.

yeah this is just a bandaid fix; the issue is certainly bound to arise in other situations. That said, I don't think it hurts to check that the file info is about an actual file rather than blindly reloading the doc

[takase1121]

The issue is moved to #1707, this PR should be merged as a hotfix.