Fix same named actor race

When an actor has already been registered with the arbiter it should
exist in the registry and thus the wait event should have been removed.
Check that the registry indeed holds an event before clearing it.

tractor/_actor.py

@@ -668,6 +668,7 @@ class Arbiter(Actor):
         events = self._waiters.pop(name, ())
         self._waiters.setdefault(name, []).append(uid)
         for event in events:
+            if isinstance(event, trio.Event):
                 event.set()
 
     def unregister_actor(self, uid: Tuple[str, str]) -> None: