Affected: crates.io: actix-files

### Summary

When passing a non-existing folder to the `actix_files::Files::new()` method causes the actix server to expose unexpected files.

### Details

The `actix-files` library exposes a [`Files` struct](https://github.com/actix/actix-web/blob/fba766b4beb92278665d58815c94d336015225c5/actix-files/src/files.rs#L38) that configures an actix `service` to serve the files in a folder as static assets. Below you can find the [signature of the `Files::new` method](https://github.com/actix/actix-web/blob/fba766b4beb92278665d58815c94d336015225c5/actix-files/src/files.rs#L98):

```rust pub fn new<T: Into<PathBuf>>(mount_path: &str, serve_from: T) -> Files ```

When the `mount_path` you pass to `Files` doesn't exist, [it defaults to an empty path](https://github.com/actix/actix-web/blob/fba766b4beb92278665d58815c94d336015225c5/actix-files/src/files.rs#L104) (`Path::new()`). When the service receives a HTTP request, it [joins the request information with the empty path](https://github.com/actix/actix-web/blob/fba766b4beb92278665d58815c94d336015225c5/actix-files/src/service.rs#L136) and calls `canonicalize`. Rust resolves this path as relative and returns any file that matches it.

This behavior causes the library to expose unexpected files when the folder is not present.

### PoC

_There is a working PoC on https://github.com/Angelmmiguel/actix-files-vuln, although the next steps can be followed to reproduce the issue_

1. Clone the https://github.com/actix/examples repository. 2. C…