Initialize options.row_cache

Summary: options.row_cache should already been initialized as null by default. Still try to set it following current convention, because one valgrind failure reports a failure related to it.

Test Plan: Run all unit tests

Reviewers: yhchiang, kradhakrishnan, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D51303

Fix clang build

Summary: Fix clang

Test Plan: make check

Reviewers: sdong, yhchiang, rven

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D51417