[RocksDB Peformance Branch] A bug in PlainTable format

Summary: A bug to fix. IT's already fixed in D14457, but want to check it in sooner to unblock tests

Test Plan: plain_table_db_test

Reviewers: nkg-, haobo

Reviewed By: nkg-

CC: kailiu, leveldb

Differential Revision: https://reviews.facebook.net/D14673