prepare_send_from: clippy: Replace two unwraps

        // macro because the closure's type parameters would be unnameable.

        macro_rules! get_output {

            ($self_:expr) => {

                $self_.output.as_mut().unwrap().as_mut()

                $self_.output.as_mut().expect(BAD_POLL_MSG).as_mut()

            };

        }

        ///

        const BAD_POLL_MSG: &str =

            "future from SinkExt::prepare_send_from (SinkPrepareSendFuture) \

                 polled after returning Ready(Ok)";

        let () = match ready!(get_output!(self_).poll_ready(cx)) {

            Err(e) => {

        };

        let sendable = SinkSendable {

            output: self_.output.take().unwrap(),

            output: self_.output.take().expect(BAD_POLL_MSG),

            tw: PhantomData,

        };