Fix mock_env.cc uninitialized variable

Summary:
Mingw is complaining about uninitialized variable in mock_env.cc. e.g. https://travis-ci.org/facebook/rocksdb/jobs/240132276
The fix is to initialize the variable.
Closes https://github.com/facebook/rocksdb/pull/2428

Differential Revision: D5211306

Pulled By: yiwu-arbug

fbshipit-source-id: ee02bf0327dcea8590a2aa087f0176fecaf8621c

env/mock_env.cc

@@ -378,7 +378,7 @@ class TestMemLogger : public Logger {
       struct timeval now_tv;
       gettimeofday(&now_tv, nullptr);
       const time_t seconds = now_tv.tv_sec;
-      struct tm t;
+      struct tm t{0, 0, 0, 0, 0, 0, 0, 0, 0};
       auto ret __attribute__((__unused__)) = localtime_r(&seconds, &t);
       assert(ret);
       p += snprintf(p, limit - p,