Avoid a lump of coal (NPE) in JobProgressListener's stocking.

core/src/main/scala/org/apache/spark/ui/jobs/JobProgressListener.scala

@@ -146,12 +146,9 @@ private[spark] class JobProgressListener(val sc: SparkContext) extends SparkList
         // update duration
         y.taskTime += taskEnd.taskInfo.duration
 
-        taskEnd.taskMetrics.shuffleReadMetrics.foreach { shuffleRead =>
-          y.shuffleRead += shuffleRead.remoteBytesRead
-        }
-
-        taskEnd.taskMetrics.shuffleWriteMetrics.foreach { shuffleWrite =>
-          y.shuffleWrite += shuffleWrite.shuffleBytesWritten
+        Option(taskEnd.taskMetrics).foreach { taskMetrics =>
+          taskMetrics.shuffleReadMetrics.foreach { y.shuffleRead += _.remoteBytesRead }
+          taskMetrics.shuffleWriteMetrics.foreach { y.shuffleWrite += _.shuffleBytesWritten }
         }
       }
       case _ => {}