[FIX] Tiki print pages : Fix the error that occurs when adding a page to the list of selected pages.

The error occurs on the in_array method at line 131, which expects a table as the second parameter while when submitting the form while trying to add a page, the $printstructures variable is NULL.

Related Ticket : https://dev.tiki.org/Regressions-in-26x (Issue 17)