[mqtt-client] added completion condition paragraph to async functions

Summary: related to T12804

Reviewers: ivica

Reviewed By: ivica

Subscribers: miljen, iljazovic

Differential Revision: https://repo.mireo.local/D26421

include/async_mqtt5/impl/client_service.hpp

@@ -196,9 +196,10 @@ public:
 
 	template <typename Authenticator>
 	void authenticator(Authenticator&& authenticator) {
-		_stream_context.authenticator(
-			std::forward<Authenticator>(authenticator)
-		);
+		if (!is_open())
+			_stream_context.authenticator(
+				std::forward<Authenticator>(authenticator)
+			);
 	}
 
 	template <typename Prop>