Fix race-condition which could lead to a NPE or Exception while register a Channel with a Worker. See #469

src/main/java/org/jboss/netty/channel/socket/nio/AbstractNioWorker.java

@@ -143,6 +143,7 @@ abstract class AbstractNioWorker implements Worker {
         assert offered;
 
         if (wakenUp.compareAndSet(false, true)) {
+            synchronized (startStopLock) {
                 // wake up the selector to speed things
                 selector = this.selector;
 
@@ -152,6 +153,8 @@ abstract class AbstractNioWorker implements Worker {
                     selector.wakeup();
                 }
             }
+
+        }
     }
 
     /**