Correctly check for double inclusion in captcha garbage collection

(Just hosing my apache error logs for new errors after 305->307-PL1 upgrade.)

This code is only called on session garbage collect but there is a conditional load of the class phpbb_captcha_factory prior to calling phpbb_captcha_factory::garbage_collect. The only prob is that your test @ line 980 is

if (!class_exists('captcha_factory'))

not

if (!class_exists('phpbb_captcha_factory'))

//Terry