[ticket/11578] Add missing underscore after 'validate' function prefix

The underscore after the 'validate' function prefix for the older
functions was dropped by accident in PR #1407. This patch will add it
back.

PHPBB3-11578
Merge remote-tracking branch 'marc1706/ticket/11578' into develop-olympus

* marc1706/ticket/11578:
  [ticket/11578] Add missing underscore after 'validate' function prefix