back-port #19181 to branch-2.2.

## What changes were proposed in this pull request?
1. a test reproducing [SPARK-21907](https://issues.apache.org/jira/browse/SPARK-21907)
2. a fix for the root cause of the issue.

`org.apache.spark.util.collection.unsafe.sort.UnsafeExternalSorter.spill` calls `org.apache.spark.util.collection.unsafe.sort.UnsafeInMemorySorter.reset` which may trigger another spill,
when this happens the `array` member is already de-allocated but still referenced by the code, this causes the nested spill to fail with an NPE in `org.apache.spark.memory.TaskMemoryManager.getPage`.
This patch introduces a reproduction in a test case and a fix, the fix simply sets the in-mem sorter's array member to an empty array before actually performing the allocation. This prevents the spilling code from 'touching' the de-allocated array.

## How was this patch tested?
introduced a new test case: `org.apache.spark.util.collection.unsafe.sort.UnsafeExternalSorterSuite#testOOMDuringSpill`.

Author: Eyal Farago <eyal@nrgene.com>

Closes #19481 from eyalfa/SPARK-21907__oom_during_spill__BACKPORT-2.2.