U16String::pop_char panic with surrogate string

[touilleMan]

#[test]
fn truncated_with_surrogate() {
    // Character U+24B62, encoded as D852 DF62 in UTF16
    let buf= "𤭢";
    let mut s = widestring::U16String::from_str(buf);
    s.pop_char();
}

output:

thread 'windows::mount::tests::truncated_with_surrogate' panicked at C:\Users\gbleu\.cargo\registry\src\index.crates.io-6f17d22bba15001f\widestring-1.0.2\src\ustring.rs:1286:42:
index out of bounds: the len is 1 but the index is 1
stack backtrace:
   0: std::panicking::begin_panic_handler
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library\std\src\panicking.rs:645
   1: core::panicking::panic_fmt
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library\core\src\panicking.rs:72
   2: core::panicking::panic_bounds_check
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library\core\src\panicking.rs:190
   3: core::slice::index::impl$2::index<u16>
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112\library\core\src\slice\index.rs:258
   4: alloc::vec::impl$12::index<u16,usize,alloc::alloc::Global>
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112\library\alloc\src\vec\mod.rs:2732
   5: widestring::ustring::U16String::pop_char
             at C:\Users\gbleu\.cargo\registry\src\index.crates.io-6f17d22bba15001f\widestring-1.0.2\src\ustring.rs:1286
   6: libparsec_platform_mountpoint::windows::mount::tests::truncated_with_surrogate
             at .\tests\unit\windows_volume_label.rs:40
   7: libparsec_platform_mountpoint::windows::mount::tests::truncated_with_surrogate::closure$0
             at .\tests\unit\windows_volume_label.rs:35
   8: core::ops::function::FnOnce::call_once<libparsec_platform_mountpoint::windows::mount::tests::truncated_with_surrogate::closure_env$0,tuple$<> >
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112\library\core\src\ops\function.rs:250
   9: core::ops::function::FnOnce::call_once
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library\core\src\ops\function.rs:250
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

   Canceling due to test failure: 0 tests still running

I guess this is due to a off-by-one issue here:

widestring-rs/src/ustring.rs

Line 1286 in e123e39

let high = self.inner[self.len()];

(should be self.inner[self.len() - 1] )