runtime: move is_join_waker_set assertion in unset_waker (#7072)

Author: Darksonn

tokio/src/runtime/task/state.rs

@@ -446,12 +446,15 @@ impl State {
     pub(super) fn unset_waker(&self) -> UpdateResult {
         self.fetch_update(|curr| {
             assert!(curr.is_join_interested());
-            assert!(curr.is_join_waker_set());
 
             if curr.is_complete() {
                 return None;
             }
 
+            // If the task is completed, this bit may have been unset by
+            // `unset_waker_after_complete`.
+            assert!(curr.is_join_waker_set());
+
             let mut next = curr;
             next.unset_join_waker();