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

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)
```

Author: Carson Wang <carson.wang@intel.com>

Closes #10637 from carsonwang/FixNPE.

core/src/main/java/org/apache/spark/util/collection/unsafe/sort/UnsafeInMemorySorter.java

@@ -116,8 +116,10 @@ public UnsafeInMemorySorter(
    * Free the memory used by pointer array.
    */
   public void free() {
-    consumer.freeArray(array);
-    array = null;
+    if (consumer != null) {
+      consumer.freeArray(array);
+      array = null;
+    }
   }
 
   public void reset() {