function `has()` would panic without pre-checking

[cicilzx]

Hi, I'm developing a fuzzer to test the safe abstraction, and I fonnd a panic bug when calling function has(). Bellow is a test case:

#[allow(warnings, unused, non_snake_case)]
#[test]
fn test_has_3lk9v() {
    let capability = String::from("\0:");
    let _ = has(capability.as_str());
}

Output:

thread 'test_has_3lk9v' panicked at src/lib.rs:1203:47:
called `Result::unwrap()` on an `Err` value: NulError(0, [0, 58])
stack backtrace:
   0: rust_begin_unwind
             at /rustc/d84b9037541f45dc2c52a41d723265af211c0497/library/std/src/panicking.rs:652:5
   1: core::panicking::panic_fmt
             at /rustc/d84b9037541f45dc2c52a41d723265af211c0497/library/core/src/panicking.rs:72:14
   2: core::result::unwrap_failed
             at /rustc/d84b9037541f45dc2c52a41d723265af211c0497/library/core/src/result.rs:1658:5
   3: core::result::Result<T,E>::unwrap
             at /rustc/d84b9037541f45dc2c52a41d723265af211c0497/library/core/src/result.rs:1081:23
   4: zmq::has
             at ./src/lib.rs:1203:17
   5: zmq::test_has_3lk9v
             at ./src/lib.rs:1367:13
   6: zmq::test_has_3lk9v::{{closure}}
             at ./src/lib.rs:1354:20
   7: core::ops::function::FnOnce::call_once
             at /rustc/d84b9037541f45dc2c52a41d723265af211c0497/library/core/src/ops/function.rs:250:5
   8: core::ops::function::FnOnce::call_once
             at /rustc/d84b9037541f45dc2c52a41d723265af211c0497/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

I think checking that &str is legal before calling ffi::CString would make the API more robust.