Fix a sign and unsign comparating in plain_table_db_test

Summary:

Test Plan:

Reviewers:

CC:

Task ID: #

Blame Rev:
Fix a bug in IterKey

Summary: IterKey set buffer_size_ to a wrong initial value, causing it to always allocate values from heap instead of stack if the key size is smaller. Fix it.

Test Plan: make all check

Reviewers: haobo, ljin

Reviewed By: haobo

CC: igor, dhruba, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D18279