Fix NullPointerException onSuccess callback

On an exception during the `Driver.process`, `driverContext.failed` is invoked. Then through the state changed callback,`TaskHandle.destory` is invoked, so all the running and queued splits that belong to the task are closed finally. As the result, there would be in the situation that "closed called before creating driver". 

The issue here is that `onSuccess` callback is still invoked in the situation. We should check spitRunner's driver is created or not as like `onFailure` callback.

```
2014-06-10T00:27:45.254+0000	 INFO	task-notification-0	stderr	java.lang.NullPointerException
2014-06-10T00:27:45.254+0000	 INFO	task-notification-0	stderr		at com.facebook.presto.execution.SqlTaskExecution$3.onSuccess(SqlTaskExecution.java:421)
2014-06-10T00:27:45.254+0000	 INFO	task-notification-0	stderr		at com.google.common.util.concurrent.Futures$4.run(Futures.java:1181)
2014-06-10T00:27:45.254+0000	 INFO	task-notification-0	stderr		at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
2014-06-10T00:27:45.255+0000	 INFO	task-notification-0	stderr		at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
2014-06-10T00:27:45.255+0000	 INFO	task-notification-0	stderr		at java.lang.Thread.run(Thread.java:744)
```