Use-after-free bugs in the code.

[quitbug]

Hi, I hv noticed that there are several dangling pointer used in the pub register_kbd_from_rmlvo function, provided in this crate.

wayland-kbd/src/mapped_keyboard.rs

Lines 390 to 413 in 05e9b55

	pub fn register_kbd_from_rmlvo<ID: 'static>(evqh: &mut EventQueueHandle, kbd: &WlKeyboard,
	implem: MappedKeyboardImplementation<ID>, idata: ID,
	rmlvo: RMLVO)
	-> Result<(), MappedKeyboardError> {
	let mut mapped_kbd = KbState::new()?;
	fn to_cstring(s: Option<String>) -> Result<Option<CString>, MappedKeyboardError> {
	s.map_or(Ok(None), |s| CString::new(s).map(Option::Some))
	.map_err(|_| MappedKeyboardError::BadNames)
	}
	let rules = to_cstring(rmlvo.rules)?;
	let model = to_cstring(rmlvo.model)?;
	let layout = to_cstring(rmlvo.layout)?;
	let variant = to_cstring(rmlvo.variant)?;
	let options = to_cstring(rmlvo.options)?;
	let xkb_names = ffi::xkb_rule_names {
	rules: rules.map_or(ptr::null(), |s| s.as_ptr()),
	model: model.map_or(ptr::null(), |s| s.as_ptr()),
	layout: layout.map_or(ptr::null(), |s| s.as_ptr()),
	variant: variant.map_or(ptr::null(), |s| s.as_ptr()),
	options: options.map_or(ptr::null(), |s| s.as_ptr()),
	};

The problematic code is from line 408 to line 412, passing dangling pointers to FFI calls after their CString data are already dropped, which are use-after-free bugs.