Don't let invoker units be posted after the invoker has been shutdown.

git-svn-id: https://samskivert.googlecode.com/svn/trunk@2290 6335cc39-0255-0410-8fd6-9bcaacd3b74c

src/java/com/samskivert/util/Invoker.java

@@ -136,6 +136,9 @@ public class Invoker extends LoopingThread
      */
     public void postUnit (Unit unit)
     {
+        if (!isRunning()) {
+            throw new IllegalStateException("Invoker has been shutdown");
+        }
         // note the time
         unit.queueStamp = System.currentTimeMillis();
         // and append it to the queue