ARTEMIS-1601 deal with race in Stomp sendMessage

If the Stomp consumer was closed at or near the same time a message was
dispatched then an NPE might result. Throwing an exception is a
relatively expensive operation in the JVM because of the stacktrace
information that needs to be generated, so in cases where it is known
that a null value could be returned one should check and handle it
appropriately.

artemis-protocols/artemis-stomp-protocol/src/main/java/org/apache/activemq/artemis/core/protocol/stomp/StompSession.java

@@ -138,6 +138,9 @@ public class StompSession implements SessionCallback {
       ICoreMessage newServerMessage = serverMessage.toCore();
       try {
          StompSubscription subscription = subscriptions.get(consumer.getID());
+         // subscription might be null if the consumer was closed
+         if (subscription == null)
+            return 0;
          StompFrame frame;
          ActiveMQBuffer buffer;