Update kunenadiscuss.php - allow guest to post first message

Kunena Discuss 3.0 Plugin does not allow guests to be the first to post article comments. 
Solution is described here: http://www.kunena.org/forum/k-3-0-kunena-discuss-p/127670-kunena-discuss-3-0-plugin-does-not-allow-guests-to-be-the-first-to-post-article-comments#144459