Fix intermittent NPE caused by session cleanup after a member leaves cluster

git-svn-id: http://svn.igniterealtime.org/svn/repos/openfire/trunk@13979 b35dd754-fafc-0310-a699-88a17e54d16e

src/plugins/hazelcast/src/java/com/jivesoftware/openfire/session/RemoteClientSession.java

@@ -150,7 +150,8 @@ public class RemoteClientSession extends RemoteSession implements ClientSession
 
     public int incrementConflictCount() {
         ClusterTask task = getRemoteSessionTask(RemoteSessionTask.Operation.incrementConflictCount);
-        return (Integer) doSynchronousClusterTask(task);
+        Object result = doSynchronousClusterTask(task);
+        return result == null ? 0 : (Integer) result;
     }
 
     RemoteSessionTask getRemoteSessionTask(RemoteSessionTask.Operation operation) {