Update cluster_monitor.py (#273)

parl/remote/cluster_monitor.py

@@ -103,6 +103,7 @@ class ClusterMonitor(object):
             cluster_address (str): IP address of the exited client.
         """
         self.lock.acquire()
+        if client_address in self.status['clients']:
             self.status['clients'].pop(client_address)
         self.lock.release()