Fix path validation for abstract unix socket.

[RealXuChe]

The validation of abstract unix socket is wrong, and I fixed it.
Look at the original validation code:

match (bytes.get(0), bytes.len().cmp(&sockaddr.sun_path.len())) {
    // Abstract paths don't need a null terminator
    (Some(&0), Ordering::Greater) => {
        return Err(io::Error::new(
            io::ErrorKind::InvalidInput,
            "path must be no longer than libc::sockaddr_un.sun_path",
        ));
    }
    (_, Ordering::Greater) | (_, Ordering::Equal) => {
        return Err(io::Error::new(
            io::ErrorKind::InvalidInput,
            "path must be shorter than libc::sockaddr_un.sun_path",
        ));
    }
    _ => {}
}

Abstract paths don't need a null terminator, so it's valid when it's no longer than the libc::sockaddr_un.sun_path. However, when the length of an abstract path equals to sun_path, (Some(&0), Ordering::Equal) will match on the second arm and will return an error.

I added the branch (Some(&0), _) => {} to accept such case.

[Thomasdezeeuw]

Can you add a test for this?

Also this is going to be replaced pretty soon: #1527.

[Thomasdezeeuw]

Closing in favour of #1749, which removes this code.