Further fixes to the pg_get_expr() security fix in back branches.

Further fixes to the pg_get_expr() security fix in back branches.

It now emerges that the JDBC driver expects to be able to use pg_get_expr()
on an output of a sub-SELECT.  So extend the check logic to be able to recurse
into a sub-SELECT to see if the argument is ultimately coming from an
appropriate column.  Per report from Thomas Kellerer.