Fix database connection leak when closing PDO connection
Fix database connection leak when closing PDO connection

We discovered a resource leak (database connection leak) in our
application's test suite. We investigated and discovered that, although
the test suite was calling `CI_DB_driver::close()` after each test, the
`result_id` member variable still maintained an internal reference to
the database connection, which kept the connection from being closed.

This is using pdo_sqlsrv; I have not tested other PDO drivers.

Signed-off-by: Josh Kelley <1c7c002a808dd2ddca893047961b83eabacb7f71@gmail.com>

bcit-ci/CodeIgniter,butane/CodeIgniter,bcit-ci/CodeIgniter,bcit-ci/CodeIgniter,butane/CodeIgniter,madwings/InsaneDB,madwings/InsaneDB,butane/CodeIgniter,bcit-ci/CodeIgniter,madwings/InsaneDB,bcit-ci/CodeIgniter,madwings/InsaneDB,butane/CodeIgniter,butane/CodeIgniter,madwings/InsaneDB