[SPARK-12690][Core] Fix NPE in UnsafeInMemorySorter.free()

[carsonwang]

I hit the exception below. The UnsafeKVExternalSorter does pass null as the consumer when creating an UnsafeInMemorySorter. Normally the NPE doesn't occur because the inMemSorter is set to null later and the free() method is not called. It happens when there is another exception like OOM thrown before setting inMemSorter to null. Anyway, we can add the null check to avoid it.

ERROR spark.TaskContextImpl: Error in TaskCompletionListener
java.lang.NullPointerException
        at org.apache.spark.util.collection.unsafe.sort.UnsafeInMemorySorter.free(UnsafeInMemorySorter.java:110)
        at org.apache.spark.util.collection.unsafe.sort.UnsafeExternalSorter.cleanupResources(UnsafeExternalSorter.java:288)
        at org.apache.spark.util.collection.unsafe.sort.UnsafeExternalSorter$1.onTaskCompletion(UnsafeExternalSorter.java:141)
        at org.apache.spark.TaskContextImpl$$anonfun$markTaskCompleted$1.apply(TaskContextImpl.scala:79)
        at org.apache.spark.TaskContextImpl$$anonfun$markTaskCompleted$1.apply(TaskContextImpl.scala:77)
        at scala.collection.mutable.ResizableArray$class.foreach(ResizableArray.scala:59)
        at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:47)
        at org.apache.spark.TaskContextImpl.markTaskCompleted(TaskContextImpl.scala:77)
        at org.apache.spark.scheduler.Task.run(Task.scala:91)
        at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:213)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
        at java.lang.Thread.run(Thread.java:722)

[SparkQA]

Test build #48918 has finished for PR 10637 at commit 2bc7827.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

cc @JoshRosen

[JoshRosen]

LGTM. Thanks @carsonwang! I'm going to merge this into master and branch-1.6.