buf_flush_init_flush_rbt() was called too early in MariaDB server 10.0,
10.1, MySQL 5.5 and MySQL 5.6. The memory leak has been fixed in
the XtraDB storage engine and in MySQL 5.7.

As a result, when the server is started to initialize new data files,
the buf_pool->flush_rbt will be created unnecessarily and then leaked.
This memory leak was noticed in MariaDB server 10.1 when running the
test encryption.innodb_first_page.