Fix handling of empty host strings

    #[corresponds(X509_VERIFY_PARAM_set1_host)]

    pub fn set_host(&mut self, host: &str) -> Result<(), ErrorStack> {

        unsafe {

            // len == 0 means "run strlen" :(

            let raw_host = if host.is_empty() { "\0" } else { host };

            cvt(ffi::X509_VERIFY_PARAM_set1_host(

                self.as_ptr(),

                host.as_ptr() as *const _,

                raw_host.as_ptr() as *const _,

                host.len(),

            ))

            .map(|_| ())