[HBASE-9815] Add extra checks to verify the NullPointer exception that was fixed by https://phabricator.fb.com/D1072451

Author: manukranthk

Summary: Adds a Preconditions check to verify if the fix.

Test Plan: Adding extra check. No code change.

Reviewers: adela, liyintang, fan

Reviewed By: adela

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D1134283

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1559395 13f79535-47bb-0310-9956-ffa450edef68