rusticl/util: add missing comment and assert to char_arr_to_cstr

I forgot to push those changes...

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33703>

src/gallium/frontends/rusticl/util/string.rs

@@ -12,9 +12,15 @@ pub fn c_string_to_string(cstr: *const c_char) -> String {
     String::from(res.unwrap_or(""))
 }
 
+/// Simple wrapper around [`CStr::from_ptr`] to bind the lifetime to a slice containing the
+/// nul-terminated string.
+///
 /// # Safety
 ///
 /// Same as [`CStr::from_ptr`]
 pub unsafe fn char_arr_to_cstr(c_str: &[c_char]) -> &CStr {
+    // We make sure that at least the end of the slice is nul-terminated. We don't really care if
+    // there is another nul within the slice.
+    debug_assert_eq!(c_str.last().copied(), Some(b'\0' as c_char));
     unsafe { CStr::from_ptr(c_str.as_ptr()) }
 }