Cookie type global announcemets forum tracking is broken.

Global announcements forums tracking seems to be incorrect.
Issue: if you have no unread global announcements in forum ID=x, forum ID=x is marked as unread in forumlist anyway.

Possible steps to reproduce:
1) Switch "Enable server-side topic marking" setting to "NO" in ACP/Load settings.
2) Mark all forums read under username2.
3) Create global announcement in forum ID=x under username1.
4)Foloow the unread GA topic last post URL under username2.
5) Go to the forumlist. GA forum is marked unread.