Fix valgrind error

Summary:
Valgrind is complaining because we are using hard_rate_limit (when serializing the options) without being initialized
http://our.intern.facebook.com/intern/sandcastle/3962140295/77533971/

Test Plan: run the test under valgrind

Reviewers: kradhakrishnan, yhchiang, igor, sdong

Reviewed By: sdong

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D46929

util/options.cc

@@ -102,6 +102,7 @@ ColumnFamilyOptions::ColumnFamilyOptions()
       source_compaction_factor(1),
       max_grandparent_overlap_factor(10),
       soft_rate_limit(0.0),
+      hard_rate_limit(0.0),
       hard_pending_compaction_bytes_limit(0),
       rate_limit_delay_max_milliseconds(1000),
       arena_block_size(0),