utils: Do not strip trailing nul from wide strings

DXC (all the Windows string types using WCHAR under the hood, really)
require wide strings to be terminated with a NUL character. If not,
they'd use a BSTR that has a sneaky length prefix (#11), otherwise the
length or ending of the string is unknown.

This has not caused any trouble before because into_vec performs the
exact same conversion as into_vec_with_nul, *except* that it pops off
the last character before returning the Vec. Only the size of it is
reduced, the underlying memory is not reallocated and the final NUL byte
is left in place.
However, in the event that a vector is shuffled around or modified we
cannot permit ourselves to loose that trailing NUL.

src/utils.rs

@@ -8,7 +8,9 @@ use thiserror::Error;
 use winapi::um::oleauto::SysStringLen;
 
 pub(crate) fn to_wide(msg: &str) -> Vec<WCHAR> {
-    widestring::WideCString::from_str(msg).unwrap().into_vec()
+    widestring::WideCString::from_str(msg)
+        .unwrap()
+        .into_vec_with_nul()
 }
 
 pub(crate) fn from_wide(wide: LPWSTR) -> String {