Panic when calling set_host with None on URL

[frewsxcv]

let mut url = Url::parse("https://example.net/hello").unwrap();
url.set_host(None).unwrap();

thread 'main' panicked at 'begin <= end (19 <= 6) when slicing `https://example.net/hello`', ../src/libcore/str/mod.rs:1719
stack backtrace:
   1:        0x10d66ffaa - std::sys::imp::backtrace::tracing::imp::write::h383d0c200a1dd210
   2:        0x10d67fd6f - std::panicking::default_hook::{{closure}}::h9445b5c503f9824b
   3:        0x10d67cf2f - std::panicking::default_hook::h36fefcc1f50aa494
   4:        0x10d67d566 - std::panicking::rust_panic_with_hook::hf54dd023dee7ef13
   5:        0x10d67d404 - std::panicking::begin_panic::ha78ff90bed4768ad
   6:        0x10d67d322 - std::panicking::begin_panic_fmt::hc7561fdea0f5f71c
   7:        0x10d67d287 - rust_begin_unwind
   8:        0x10d6bfc10 - core::panicking::panic_fmt::hb7aed24f08b1bad6
   9:        0x10d6c0ae5 - core::str::slice_error_fail::hc148aa73da59a300
  10:        0x10d4d3314 - core::str::traits::<impl core::ops::Index<core::ops::Range<usize>> for str>::index::hc91b0c453a2b9a37
  11:        0x10d4c9474 - <collections::string::String as core::ops::Index<core::ops::Range<usize>>>::index::hfbbcd72b44dfd783
  12:        0x10d4caaba - collections::string::String::drain::h1868f57c1d4390c5
  13:        0x10d4ee1e2 - url::Url::set_host::hcdae4ea78ed9facd
  14:        0x10d4c8ca9 - rust_out::main::ha208b69ccbc11839
  15:        0x10d68032a - __rust_maybe_catch_panic
  16:        0x10d67ddf6 - std::rt::lang_start::h26c24604fceba8d3
  17:        0x10d4c8d89 - main
', ../src/librustdoc/test.rs:315
stack backtrace:
   1:        0x113674faa - std::sys::imp::backtrace::tracing::imp::write::h383d0c200a1dd210
   2:        0x113684d6f - std::panicking::default_hook::{{closure}}::h9445b5c503f9824b
   3:        0x113681e35 - std::panicking::default_hook::h36fefcc1f50aa494
   4:        0x113682566 - std::panicking::rust_panic_with_hook::hf54dd023dee7ef13
   5:        0x113682404 - std::panicking::begin_panic::ha78ff90bed4768ad
   6:        0x113682322 - std::panicking::begin_panic_fmt::hc7561fdea0f5f71c
   7:        0x10f0deaf5 - rustdoc::test::runtest::h9f5d35a4e456f3f0
   8:        0x10efcd420 - <F as test::FnBox<T>>::call_box::hfb4e687f64168832
   9:        0x11368532a - __rust_maybe_catch_panic
  10:        0x10f33bb50 - std::panicking::try::do_call::h7476444272edf7f4
  11:        0x11368532a - __rust_maybe_catch_panic
  12:        0x10f341f9a - <F as alloc::boxed::FnBox<A>>::call_box::hf1758710039efd69
  13:        0x1136813d4 - std::sys::imp::thread::Thread::new::thread_start::hcd65577217ddb0d9
  14:     0x7fffea8beaaa - _pthread_body
  15:     0x7fffea8be9f6 - _pthread_start