[SPARK-21133][CORE] Fix HighlyCompressedMapStatus#writeExternal throws NPE

## What changes were proposed in this pull request?

Fix HighlyCompressedMapStatus#writeExternal NPE:
```
17/06/18 15:00:27 ERROR Utils: Exception encountered
java.lang.NullPointerException
        at org.apache.spark.scheduler.HighlyCompressedMapStatus$$anonfun$writeExternal$2.apply$mcV$sp(MapStatus.scala:171)
        at org.apache.spark.scheduler.HighlyCompressedMapStatus$$anonfun$writeExternal$2.apply(MapStatus.scala:167)
        at org.apache.spark.scheduler.HighlyCompressedMapStatus$$anonfun$writeExternal$2.apply(MapStatus.scala:167)
        at org.apache.spark.util.Utils$.tryOrIOException(Utils.scala:1303)
        at org.apache.spark.scheduler.HighlyCompressedMapStatus.writeExternal(MapStatus.scala:167)
        at java.io.ObjectOutputStream.writeExternalData(ObjectOutputStream.java:1459)
        at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1430)
        at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
        at java.io.ObjectOutputStream.writeArray(ObjectOutputStream.java:1378)
        at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1174)
        at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
        at org.apache.spark.MapOutputTracker$$anonfun$serializeMapStatuses$1.apply$mcV$sp(MapOutputTracker.scala:617)
        at org.apache.spark.MapOutputTracker$$anonfun$serializeMapStatuses$1.apply(MapOutputTracker.scala:616)
        at org.apache.spark.MapOutputTracker$$anonfun$serializeMapStatuses$1.apply(MapOutputTracker.scala:616)
        at org.apache.spark.util.Utils$.tryWithSafeFinally(Utils.scala:1337)
        at org.apache.spark.MapOutputTracker$.serializeMapStatuses(MapOutputTracker.scala:619)
        at org.apache.spark.MapOutputTrackerMaster.getSerializedMapOutputStatuses(MapOutputTracker.scala:562)
        at org.apache.spark.MapOutputTrackerMaster$MessageLoop.run(MapOutputTracker.scala:351)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:745)
17/06/18 15:00:27 ERROR MapOutputTrackerMaster: java.lang.NullPointerException
java.io.IOException: java.lang.NullPointerException
        at org.apache.spark.util.Utils$.tryOrIOException(Utils.scala:1310)
        at org.apache.spark.scheduler.HighlyCompressedMapStatus.writeExternal(MapStatus.scala:167)
        at java.io.ObjectOutputStream.writeExternalData(ObjectOutputStream.java:1459)
        at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1430)
        at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
        at java.io.ObjectOutputStream.writeArray(ObjectOutputStream.java:1378)
        at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1174)
        at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
        at org.apache.spark.MapOutputTracker$$anonfun$serializeMapStatuses$1.apply$mcV$sp(MapOutputTracker.scala:617)
        at org.apache.spark.MapOutputTracker$$anonfun$serializeMapStatuses$1.apply(MapOutputTracker.scala:616)
        at org.apache.spark.MapOutputTracker$$anonfun$serializeMapStatuses$1.apply(MapOutputTracker.scala:616)
        at org.apache.spark.util.Utils$.tryWithSafeFinally(Utils.scala:1337)
        at org.apache.spark.MapOutputTracker$.serializeMapStatuses(MapOutputTracker.scala:619)
        at org.apache.spark.MapOutputTrackerMaster.getSerializedMapOutputStatuses(MapOutputTracker.scala:562)
        at org.apache.spark.MapOutputTrackerMaster$MessageLoop.run(MapOutputTracker.scala:351)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException
        at org.apache.spark.scheduler.HighlyCompressedMapStatus$$anonfun$writeExternal$2.apply$mcV$sp(MapStatus.scala:171)
        at org.apache.spark.scheduler.HighlyCompressedMapStatus$$anonfun$writeExternal$2.apply(MapStatus.scala:167)
        at org.apache.spark.scheduler.HighlyCompressedMapStatus$$anonfun$writeExternal$2.apply(MapStatus.scala:167)
        at org.apache.spark.util.Utils$.tryOrIOException(Utils.scala:1303)
        ... 17 more
17/06/18 15:00:27 INFO MapOutputTrackerMasterEndpoint: Asked to send map output locations for shuffle 0 to 10.17.47.20:50188
17/06/18 15:00:27 ERROR Utils: Exception encountered
java.lang.NullPointerException
        at org.apache.spark.scheduler.HighlyCompressedMapStatus$$anonfun$writeExternal$2.apply$mcV$sp(MapStatus.scala:171)
        at org.apache.spark.scheduler.HighlyCompressedMapStatus$$anonfun$writeExternal$2.apply(MapStatus.scala:167)
        at org.apache.spark.scheduler.HighlyCompressedMapStatus$$anonfun$writeExternal$2.apply(MapStatus.scala:167)
        at org.apache.spark.util.Utils$.tryOrIOException(Utils.scala:1303)
        at org.apache.spark.scheduler.HighlyCompressedMapStatus.writeExternal(MapStatus.scala:167)
        at java.io.ObjectOutputStream.writeExternalData(ObjectOutputStream.java:1459)
        at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1430)
        at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
        at java.io.ObjectOutputStream.writeArray(ObjectOutputStream.java:1378)
        at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1174)
        at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
        at org.apache.spark.MapOutputTracker$$anonfun$serializeMapStatuses$1.apply$mcV$sp(MapOutputTracker.scala:617)
        at org.apache.spark.MapOutputTracker$$anonfun$serializeMapStatuses$1.apply(MapOutputTracker.scala:616)
        at org.apache.spark.MapOutputTracker$$anonfun$serializeMapStatuses$1.apply(MapOutputTracker.scala:616)
        at org.apache.spark.util.Utils$.tryWithSafeFinally(Utils.scala:1337)
        at org.apache.spark.MapOutputTracker$.serializeMapStatuses(MapOutputTracker.scala:619)
        at org.apache.spark.MapOutputTrackerMaster.getSerializedMapOutputStatuses(MapOutputTracker.scala:562)
        at org.apache.spark.MapOutputTrackerMaster$MessageLoop.run(MapOutputTracker.scala:351)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:745)
```

## How was this patch tested?

manual tests

Author: Yuming Wang <wgyumg@gmail.com>

Closes #18343 from wangyum/SPARK-21133.