Panic for `file` while using base

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

Describe the bug

Testcase:

#[test]
fn test_file_base_panic() {
    let base_url = Url::parse("file:///w:/a:").unwrap();
    Url::options().base_url(Some(&base_url)).parse("file:..");
}

Backtrace:

thread 'test_file_base_panic' (377458) panicked at url/src/parser.rs:1321:21:
assertion failed: self.serialization.as_bytes()[segment_start - 1] == b'/'
stack backtrace:
< skipped >
  19:     0x57a3a138611c - core::panicking::panic::hb515f22c0e98143a
                               at /rustc/f8297e351a40c1439a467bbbb6879088047f50b3/library/core/src/panicking.rs:145:5
  20:     0x57a3a12cfb59 - url::parser::Parser::parse_path::h69b46743c9367ec2
                               at /home/wooffie/work/upstream/rust-url/url/src/parser.rs:1321:21
  21:     0x57a3a12cd0d9 - url::parser::Parser::parse_file::h88e81c17dec524c5
                               at /home/wooffie/work/upstream/rust-url/url/src/parser.rs:659:46
  22:     0x57a3a12d3896 - url::parser::Parser::parse_with_scheme::h31ca21d8147c599c
                               at /home/wooffie/work/upstream/rust-url/url/src/parser.rs:446:22
  23:     0x57a3a12d637d - url::parser::Parser::parse_url::hadfb176b92197e15
                               at /home/wooffie/work/upstream/rust-url/url/src/parser.rs:382:25
  24:     0x57a3a12af420 - url::ParseOptions::parse::hf1b29cfaa662d5f3
                               at /home/wooffie/work/upstream/rust-url/url/src/lib.rs:314:10
  25:     0x57a3a124735b - unit::test_file_base_panic::h46e4b9e042615051
                               at /home/wooffie/work/upstream/rust-url/url/tests/unit.rs:725:46

ref #838

Start in url/src/parser.rs at Parser::parse_file and Parser::parse_path, using the file-base testcase from the issue and the related test area in url/tests/unit.rs. Reproduce the panic, trace the path handling around the reported assertion, and add or update a regression test. Done means the testcase parses without panicking and the relevant tests pass.