Fix a bug causing LOG is not created when max_log_file_size is set.

Summary:
Fix a bug causing LOG is not created when max_log_file_size is set.
This bug is reported in issue #174.

Test Plan:
Add TEST(AutoRollLoggerTest, LogFileExistence).
make auto_roll_logger_test
./auto_roll_logger_test

Reviewers: haobo, sdong, ljin, igor, igor2

Reviewed By: igor2

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D19053