[SPARK-17056][CORE] Fix a wrong assert regarding unroll memory in MemoryStore

## What changes were proposed in this pull request?

There is an assert in MemoryStore's putIteratorAsValues method which is used to check if unroll memory is not released too much. This assert looks wrong.

## How was this patch tested?

Jenkins tests.

Author: Liang-Chi Hsieh <simonh@tw.ibm.com>

Closes #14642 from viirya/fix-unroll-memory.