Fix variable referenced before assignment

Fixes #85

Author: miguelgrinberg

socketio/asyncio_server.py

@@ -313,7 +313,7 @@ async def _trigger_event(self, event, namespace, *args):
                 try:
                     ret = await self.handlers[namespace][event](*args)
                 except asyncio.CancelledError:  # pragma: no cover
-                    pass
+                    ret = None
             else:
                 ret = self.handlers[namespace][event](*args)
             return ret