[ticket/12764] Properly handle errors upon connecting to MySQLi database

As the db_connect_id gets set up by mysql_init(), the db_connect_id will be an
object with empty settings instead of just empty. Even if mysql_real_connect()
encounters an error upon connecting, the db_connect_id is still set. This
will result in trying to just access the database which obviously does
nothing.
By setting db_connect_id to an empty string, the script will not try to query
th database and properly handle any errors that occur upon connecting.

Ticket: https://tracker.phpbb.com/browse/PHPBB3-12764