[ticket/11720] Do not call $captcha->validate if $captcha is not set

PHPBB3-11566 changed big parts of code. Unfortunately, a call to
$captcha->validate was added that is being called even if $captcha hasn't
been initialized. This change will fix this issue.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-11720

PHPBB3-11720