ensure control socket is connected before using it

`_control_socket` can be None if shutdown_kernel is called twice somehow.

Author: minrk

jupyter_client/manager.py

@@ -254,6 +254,8 @@ def request_shutdown(self, restart=False):
         """
         content = dict(restart=restart)
         msg = self.session.msg("shutdown_request", content=content)
+        # ensure control socket is connected
+        self._connect_control_socket()
         self.session.send(self._control_socket, msg)
 
     def finish_shutdown(self, waittime=None, pollinterval=0.1):