Panic in `Url::set_host`

While working on creating a more extensive fuzzing target as in https://github.com/servo/rust-url/issues/654 I found this panic:

#[test]
    let mut url = Url::parse("\nx--n://\u{b}#.").unwrap();
    dbg!(&url);
    /*
    Url {
        scheme: "x--n",
        host: Some(
            Domain(
                "%0B",
            ),
        ),
        port: None,
        path: "",
        query: None,
        fragment: Some(
            ".",
        ),
        username: "",
        password: None,
    }
    */
    url.set_host(None).unwrap();

panicked at 'assertion failed: self.byte_at(self.path_start) == b'/'', url/src/lib.rs:1685:13

Start with the reproducer in the issue and inspect url/src/lib.rs around line 1685, where Url::set_host(None) triggers the assertion. Trace how the parsed URL's host, path, and path_start are represented before set_host runs. Done means the reproducer no longer panics and a regression test covers this input.