Allow oneshot::Receiver::close after successful try_recv

Before this commit `close` after successful `try_recv` panics.

My use case is this: on drop, I call `close` to prevent pushing a
message to the queue, and then fetch the message if any and process
it.

But if message is already processed, `close` panics.  And there is
no API to know if message was already fetched or not (except for
writing a wrapped which would track that info, which would be an
overkill).

But generally `close` operation should be safe to be called any
time.

tokio/src/sync/oneshot.rs

@@ -490,8 +490,9 @@ impl<T> Receiver<T> {
     /// }
     /// ```
     pub fn close(&mut self) {
-        let inner = self.inner.as_ref().unwrap();
-        inner.close();
+        if let Some(inner) = self.inner.as_ref() {
+            inner.close();
+        }
     }
 
     /// Attempts to receive a value.

tokio/tests/sync_oneshot.rs

@@ -180,6 +180,16 @@ fn close_try_recv_poll() {
     let _ = rx.poll();
 }
 
+#[test]
+fn close_after_recv() {
+    let (tx, mut rx) = oneshot::channel::<i32>();
+
+    tx.send(17).unwrap();
+
+    assert_eq!(17, rx.try_recv().unwrap());
+    rx.close();
+}
+
 #[test]
 fn drops_tasks() {
     let (mut tx, mut rx) = oneshot::channel::<i32>();