[SPARK-11424] Guard against double-close() of RecordReaders

**TL;DR**: We can rule out one rare but potential cause of input stream corruption via defensive programming.

## Background

[MAPREDUCE-5918](https://issues.apache.org/jira/browse/MAPREDUCE-5918) is a bug where an instance of a decompressor ends up getting placed into a pool multiple times. Since the pool is backed by a list instead of a set, this can lead to the same decompressor being used in different places at the same time, which is not safe because those decompressors will overwrite each other's buffers. Sometimes this buffer sharing will lead to exceptions but other times it will might silently result in invalid / garbled input.

That Hadoop bug is fixed in Hadoop 2.7 but is still present in many Hadoop versions that we wish to support. As a result, I think that we should try to work around this issue in Spark via defensive programming to prevent RecordReaders from being closed multiple times.

So far, I've had a hard time coming up with explanations of exactly how double-`close()`s occur in practice, but I do have a couple of explanations that work on paper.

For instance, it looks like https://github.com/apache/spark/pull/7424, added in 1.5, introduces at least one extremely~rare corner-case path where Spark could double-close() a LineRecordReader instance in a way that triggers the bug. Here are the steps involved in the bad execution that I brainstormed up:

* [The task has finished reading input, so we call close()](https://github.com/apache/spark/blob/v1.5.1/core/src/main/scala/org/apache/spark/rdd/NewHadoopRDD.scala#L168).
* [While handling the close call and trying to close the reader, reader.close() throws an exception]( https://github.com/apache/spark/blob/v1.5.1/core/src/main/scala/org/apache/spark/rdd/NewHadoopRDD.scala#L190)
* We don't set `reader = null` after handling this exception, so the [TaskCompletionListener also ends up calling NewHadoopRDD.close()](https://github.com/apache/spark/blob/v1.5.1/core/src/main/scala/org/apache/spark/rdd/NewHadoopRDD.scala#L156), which, in turn, closes the record reader again.

In this hypothetical situation, `LineRecordReader.close()` could [fail with an exception if its InputStream failed to close](https://github.com/apache/hadoop/blob/release-1.2.1/src/mapred/org/apache/hadoop/mapred/LineRecordReader.java#L212).
I googled for "Exception in RecordReader.close()" and it looks like it's possible for a closed Hadoop FileSystem to trigger an error there: [SPARK-757](https://issues.apache.org/jira/browse/SPARK-757), [SPARK-2491](https://issues.apache.org/jira/browse/SPARK-2491)

Looking at [SPARK-3052](https://issues.apache.org/jira/browse/SPARK-3052), it seems like it's possible to get spurious exceptions there when there is an error reading from Hadoop. If the Hadoop FileSystem were to get into an error state _right_ after reading the last record then it looks like we could hit the bug here in 1.5.

## The fix

This patch guards against these issues by modifying `HadoopRDD.close()` and `NewHadoopRDD.close()` so that they set `reader = null` even if an exception occurs in the `reader.close()` call. In addition, I modified `NextIterator. closeIfNeeded()` to guard against double-close if the first `close()` call throws an exception.

I don't have an easy way to test this, since I haven't been able to reproduce the bug that prompted this patch, but these changes seem safe and seem to rule out the on-paper reproductions that I was able to brainstorm up.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #9382 from JoshRosen/hadoop-decompressor-pooling-fix and squashes the following commits:

5ec97d7 [Josh Rosen] Add SqlNewHadoopRDD.unsetInputFileName() that I accidentally deleted.
ae46cf4 [Josh Rosen] Merge remote-tracking branch 'origin/master' into hadoop-decompressor-pooling-fix
087aa63 [Josh Rosen] Guard against double-close() of RecordReaders.

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