Fixing memory leaks in cuckoo_table_builder_test

Summary: Fixes some memory leaks in cuckoo_builder_test.cc. This also fixed broken valgrind_check tests

Test Plan:
make valgrind_check
./cuckoo_builder_test
Currently running make check all. I shall update once it is done.

Reviewers: ljin, sdong, yhchiang, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20385