rt: fix storing Runtime in thread-local (#2011)

Storing a `Runtime` value in a thread-local resulted in a panic due to
the inability to access the parker.

This fixes the bug by skipping parking if it fails. In general, there
isn't much that we can do besides not parking.

Fixes #593

tokio/tests/rt_common.rs

@@ -617,6 +617,24 @@ rt_test! {
         assert_ok!(drop_rx.recv());
     }
 
+    #[test]
+    fn runtime_in_thread_local() {
+        use std::cell::RefCell;
+        use std::thread;
+
+        thread_local!(
+            static R: RefCell<Option<Runtime>> = RefCell::new(None);
+        );
+
+        thread::spawn(|| {
+            R.with(|cell| {
+                *cell.borrow_mut() = Some(rt());
+            });
+
+            let _rt = rt();
+        }).join().unwrap();
+    }
+
     async fn client_server(tx: mpsc::Sender<()>) {
         let mut server = assert_ok!(TcpListener::bind("127.0.0.1:0").await);