OF-1310 Fix removal of last item of pubsub node

patch contributed by Evgeny Kuklo

src/java/org/jivesoftware/openfire/pubsub/LeafNode.java

@@ -287,6 +287,9 @@ public class LeafNode extends Node {
         // Remove deleted items from the database
         for (PublishedItem item : toDelete) {
             PubSubPersistenceManager.removePublishedItem(item);
+            if (lastPublished != null && lastPublished.getID().equals(item.getID())) {
+                lastPublished = null;
+            }
         }
         if (isNotifiedOfRetract()) {
             // Broadcast notification deletion to subscribers