Fix NPE while stopping the RabbitMQEventBus bean when there is no connection established with RabbitMQ Event Bus (apache#12635)

Author: sureshanaparti

plugins/event-bus/rabbitmq/src/main/java/org/apache/cloudstack/mom/rabbitmq/RabbitMQEventBus.java

@@ -493,7 +493,7 @@ public boolean start() {
 
     @Override
     public synchronized boolean stop() {
-        if (s_connection.isOpen()) {
+        if (s_connection != null && s_connection.isOpen()) {
             for (String subscriberId : s_subscribers.keySet()) {
                 Ternary<String, Channel, EventSubscriber> subscriberDetails = s_subscribers.get(subscriberId);
                 Channel channel = subscriberDetails.second();