Fix uninitialized parameter caused by D24513

Summary: D24513 introduced a bug that a variable is not initialized. It also causes valgrind issue.

Test Plan: Run tests used to fail valgrind and make sure it passes

Reviewers: yhchiang, ljin, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D25569

db/internal_stats.h

@@ -279,6 +279,7 @@ class InternalStats {
           write_with_wal(0),
           write_other(0),
           write_self(0),
+          num_keys_written(0),
           seconds_up(0) {}
   } db_stats_snapshot_;