panic on unwrap of empty string#196

Hi I did some fuzzing of a library that uses this library, and found this issue: ``` called `Result::unwrap()` on an `Err` value: NulError(0, [0]) thread 'tests::test_to_c_str' panicked at 'called `Result::unwrap()` on an `Err` value: NulError(0, [0])', src/lib.rs:65:29 ``` It can be replicated with this unit test. ``` #[cfg(test)] mod tests { use std::str; use ToCStr; #[test] fn test_to_c_str() { let null : Vec<u8> = vec![0]; let null_str = str::from_utf8(&null).unwrap(); null_str.to_c_str(); } } ```