Bug report: assertion failure in the `index` function in the `url/src/slicing.rs` file.

• Note that this crate implements the URL Standard not RFC 1738 or RFC 3986

Describe the bug
There appears to be an assertion failure in the index function in the url/src/slicing.rs file.

Error message and stack backtrace

running 1 test
test fast_test2 ... FAILED

successes:

successes:

failures:

---- fast_test2 stdout ----
test="http://userDpass@mozilla.org"
sliced: ""
sliced: "http"
sliced: "http://"
sliced: "http://userDpass"
thread 'fast_test2' panicked at url/src/slicing.rs:147:21:
assertion failed: self.username_end == self.host_start
stack backtrace:
   0: rust_begin_unwind
             at /rustc/5069856495870486134dd2ca0b0e2516308c5c2a/library/std/src/panicking.rs:665:5
   1: core::panicking::panic_fmt
             at /rustc/5069856495870486134dd2ca0b0e2516308c5c2a/library/core/src/panicking.rs:74:14
   2: core::panicking::panic
             at /rustc/5069856495870486134dd2ca0b0e2516308c5c2a/library/core/src/panicking.rs:148:5
   3: url::slicing::<impl url::Url>::index
             at ./src/slicing.rs:147:21
   4: url::slicing::<impl core::ops::index::Index<core::ops::range::Range<url::slicing::Position>> for url::Url>::index
             at ./src/slicing.rs:36:54
   5: fuzz_test2::fast_test2
             at ./tests/fuzz_test2.rs:120:33
   6: fuzz_test2::fast_test2::{{closure}}
             at ./tests/fuzz_test2.rs:90:16
   7: core::ops::function::FnOnce::call_once
             at /rustc/5069856495870486134dd2ca0b0e2516308c5c2a/library/core/src/ops/function.rs:250:5
   8: core::ops::function::FnOnce::call_once
             at /rustc/5069856495870486134dd2ca0b0e2516308c5c2a/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.


failures:
    fast_test2

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 2 filtered out; finished in 0.08s

error: test failed, to rerun pass `-p url --test fuzz_test2`

How to reproduce

1. select any inputs in the example below
2. run the test

    let data = "http://userDpass@mozilla.org";
    // let data = "httM://userpQass@mozilla.org";
    // let data = "https://username;password@www.eample.c/m:8080/p)ath/to/opage?query=example#fragment";
    // let data = "https://usernamepassword@wbww.examplencom:080/path/to/page?query=example\"fragmenkt";
    // let data = "https://usernamepassword@www.example.com:8280/path/to/page?query=example#fragment";
    println!("test={:?}", data);
    if let Ok(url) = Url::parse(data) {
        let positions = [
            Position::BeforeScheme,
            Position::AfterScheme,
            Position::BeforeUsername,
            Position::AfterUsername,
            Position::BeforePassword,
            Position::AfterPassword,
            Position::BeforeHost,
            Position::AfterHost,
            Position::BeforePort,
            Position::AfterPort,
            Position::BeforePath,
            Position::AfterPath,
            Position::BeforeQuery,
            Position::AfterQuery,
            Position::BeforeFragment,
            Position::AfterFragment,
        ];

        // Attempt to create slices of the URL using all "good" combinations of positions
        for (i, &start) in positions.iter().enumerate() {
            for &end in &positions[i..] {
                let slice = &url[start..end];
                println!("sliced: {:?}", slice);
            }
        }
    } else {
        println!("Invalid URL");
    }

Start in url/src/slicing.rs at Url::index and reproduce the assertion with the input shown in the issue. Run the fast_test2 case in tests/fuzz_test2.rs and inspect the reported position relationships. Done means the listed URL slices no longer trigger the assertion while the test continues to pass.