fixed NPE under some abnormal shut down situation

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@15942 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/model/Hudson.java

@@ -1915,7 +1915,8 @@ public final class Hudson extends Node implements ItemGroup<TopLevelItem>, Stapl
             c.kill();
             pending.add(c.disconnect());
         }
-        udpBroadcastThread.shutdown();
+        if(udpBroadcastThread!=null)
+            udpBroadcastThread.shutdown();
         ExternalJob.reloadThread.interrupt();
         Trigger.timer.cancel();
         // TODO: how to wait for the completion of the last job?