set_path panics on oversized input instead of returning an error

[xc01]

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

Describe the bug

Summary

Url::set_path can panic on extremely large input due to internal arithmetic overflow (in restore_after_path) instead of returning an error.

Reproduction

use url::Url;

fn main() {
    let mut url = Url::parse("http://a/?a#b").unwrap();

    let encoded_path_bytes_target = (u32::MAX as usize) - 9;
    let input_spaces = encoded_path_bytes_target / 3;
    let huge_path = " ".repeat(input_spaces);

    url.set_path(&huge_path); // panic (with overflow checks)
}

Expected

Graceful failure (e.g., Result error) for oversized input, not panic.

[Manishearth]

Yeah, we should return an error here.