Make sure logger is safely released in `InfoLogLevel`

Summary: fix the memory leak that was captured by jenkin build.

Test Plan: ran the valgrind test locally

Reviewers: yhchiang

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16389

util/auto_roll_logger_test.cc

@@ -244,8 +244,9 @@ TEST(AutoRollLoggerTest, InfoLogLevel) {
   const int kInfoLogLevelNameLens[5] = {5, 4, 4, 5, 5};
 
   size_t log_size = 8192;
-  AutoRollLogger* logger =
+  std::unique_ptr<AutoRollLogger> logger_guard(
-      new AutoRollLogger(Env::Default(), kTestDir, "", log_size, 0);
+      new AutoRollLogger(Env::Default(), kTestDir, "", log_size, 0));
+  auto logger = logger_guard.get();
 
   int message_length = kSampleMessage.length();
   int log_length = 0;