Fix error when stop() is called on certain conditions (#70)

Fixes:
  AttributeError: "'NoneType' object has no attribute 'join'"

when calling stop and not connected (invalid password, fs is not running)

greenswitch/esl.py

@@ -216,10 +216,12 @@ def stop(self):
             except (NotConnectedError, socket.error):
                 pass
         self._run = False
-        logging.info("Waiting for receive greenlet exit")
-        self._receive_events_greenlet.join()
-        logging.info("Waiting for event processing greenlet exit")
-        self._process_events_greenlet.join()
+        if self._receive_events_greenlet:
+            logging.info("Waiting for receive greenlet exit")
+            self._receive_events_greenlet.join()
+        if self._process_events_greenlet:
+            logging.info("Waiting for event processing greenlet exit")
+            self._process_events_greenlet.join()
         self.sock.close()
         self.sock_file.close()