Fixed NPE when getting the session count and the parameter is null. JM-187

git-svn-id: http://svn.igniterealtime.org/svn/repos/messenger/trunk@1048 b35dd754-fafc-0310-a699-88a17e54d16e

src/java/org/jivesoftware/messenger/SessionManager.java

@@ -843,6 +843,9 @@ public class SessionManager extends BasicModule {
     }
 
     public int getSessionCount(String username) {
+        if (username == null) {
+            return 0;
+        }
         int sessionCount = 0;
         SessionMap sessionMap = sessions.get(username);
         if (sessionMap != null) {