Initialize all doubles to 0 in histogram.cc

Summary:
The existing code did not initialize a few doubles in histogram.cc.
Cropped up when I wrote a unit-test.

Test Plan: make all check

Reviewers: chip

Reviewed By: chip

CC: leveldb

Differential Revision: https://reviews.facebook.net/D8319

util/histogram.cc

@@ -58,6 +58,11 @@ namespace {
 
 
 Histogram::Histogram() :
+  min_(bucketMapper.LastValue()),
+  max_(0),
+  num_(0),
+  sum_(0),
+  sum_squares_(0),
   buckets_(std::vector<uint64_t>(bucketMapper.BucketCount(), 0)) {}
 
 void Histogram::Clear() {