Fix a potential race condition where the worker thread of GlobalEventExecutor terminates with a pending task

- Potential fix for the hanging SpdyFrameDecoderTest

common/src/main/java/io/netty/util/concurrent/GlobalEventExecutor.java

@@ -204,8 +204,8 @@ public final class GlobalEventExecutor extends AbstractEventExecutor {
         if (inEventLoop) {
             addTask(task);
         } else {
-            startThread();
             addTask(task);
+            startThread();
         }
     }