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

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

@@ -143,7 +143,14 @@ abstract class AbstractNioWorker implements Worker {
         assert offered;
 
         if (wakenUp.compareAndSet(false, true)) {
-            selector.wakeup();
+            // wake up the selector to speed things
+            selector = this.selector;
+
+            // Check if the selector is not null to prevent NPE if selector was
+            // set to null from another thread. See #469
+            if (selector != null) {
+                selector.wakeup();
+            }
         }
     }