Out of index error found by afl.rs

[StevenJiang1110]

I run afl.rs on url2.2.0 with the patch mentioned in #655 ，and I found new out of index error(The error should have nothing about the patch, for it can be replayed on version2.2.0 without patch). The code to replay the error is

let mut _local0 = url::Url::parse("m://").unwrap();
let mut _local1 = url::Url::path_segments_mut(&mut _local0).unwrap();
let _ = url::PathSegmentsMut::pop_if_empty(&mut _local1);

or

let mut _local0 = url::Url::parse("o://").unwrap();
let mut _local1 = url::Url::path_segments_mut(&mut _local0).unwrap();
let _ = url::PathSegmentsMut::pop(&mut _local1);

(I guess this two is about the same error)
I put the replay files and more inputs that may cause crash on
first
second
It seems the url with empty content after "//" or url with some specific unicode characters may lead to the panic.
I hope you will check if this is a real bug. Thanks a lot.

[StevenJiang1110]

The total bug report with RUST_BACKTRACE=full is like this

[djc]

Thanks for the report, keep 'em coming if there's anything more!