Regression from the ticket PHPBB3-9008 fix.
When topic marking was enabled for guests, and a guest visited a forum with
a new topic which is marked unread, the built SQL missed an alias for a
TOPICS_TABLE which resulted in the following error:
Unknown column 't.topic_approved' in 'where clause' [1054]
The fix is to add an alias for the table.
PHPBB3-10497
PHPBB3-9008