The problem was that if a prepared statement accessed a view, the
access to the tables listed in the query after that view was done in
the security context of the view.

The bug was in the assigning of the security context to the tables
belonging to a view: we traversed the list of all query tables
instead.  It didn't show up in the normal (non-prepared) statements
because of the different order of the steps of checking privileges
and descending into a view for normal and prepared statements.

The solution is to traverse the list and stop once the last table
belonging to the view was processed.