Use-after-free bugs in the pub functions

[quitbug]

Hi, I've noticed three dangling pointer issues in the below functions, which could lead to use-after-free bugs.

rust-linenoise/src/lib.rs

Lines 89 to 96 in 7ba975c

	pub fn history_add(line: &str) -> i32 {
	let cs = CString::new(line).unwrap().as_ptr();
	let ret: i32;
	unsafe {
	ret = ffi::linenoiseHistoryAdd(cs);
	}
	ret
	}

rust-linenoise/src/lib.rs

Lines 108 to 115 in 7ba975c

	pub fn history_save(file: &str) -> i32 {
	let fname = CString::new(file).unwrap().as_ptr();
	let ret: i32;
	unsafe {
	ret = ffi::linenoiseHistorySave(fname);
	}
	ret
	}

rust-linenoise/src/lib.rs

Lines 118 to 125 in 7ba975c

	pub fn history_load(file: &str) -> i32 {
	let fname = CString::new(file).unwrap().as_ptr();
	let ret: i32;
	unsafe {
	ret = ffi::linenoiseHistoryLoad(fname);
	}
	ret
	}

All three functions are due to a same line of code: let fname = CString::new(file).unwrap().as_ptr();.

Suggested fix:
put the above code in the same statement of the FFI call, like ffi::linenoiseHistorySave(CString::new(file).unwrap().as_ptr());