[SPARK-13958] Executor OOM due to unbounded growth of pointer array in…

## What changes were proposed in this pull request?

This change fixes the executor OOM which was recently introduced in PR apache/spark#11095
(Please fill in changes proposed in this fix)

## How was this patch tested?
Tested by running a spark job on the cluster.
(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)

(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

… Sorter

Author: Sital Kedia <skedia@fb.com>

Closes #11794 from sitalkedia/SPARK-13958.