Fix bloom filters

Summary: https://reviews.facebook.net/D13167 broke bloom filters. If filter is not in cache, we want to return true (safe thing). Am I right?

Test Plan: when benchmarking https://reviews.facebook.net/D14031 I got different results when using bloom filters vs. when not using them. This fixed the issue. I will also be putting this change to the other diff, but that one will probably be in review for longer time.

Reviewers: kailiu, dhruba, haobo

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14085