Fix valgrind error in c_test

Summary:
External contribution caused some valgrind errors: https://github.com/facebook/rocksdb/commit/1a34aaaef0900785c2de7e55b55d8c48d1201300

This diff fixes them

Test Plan: ran valgrind

Reviewers: sdong, yhchiang, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19485