[RocksDB] fix prefix_test

Summary: user comparator needs to work if either input is prefix only.

Test Plan: ./prefix_test --write_buffer_size=100000 --total_prefixes=10000 --items_per_prefix=10

Reviewers: dhruba, igor

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14241