fix: Forum/Thread: online users: invisible users < 2 not shown, refs #1456

* showthread.php / forumdisplay.php
  * fixed wrong conditional catching the case that the user is invisible himself
  * fixed the counter for invisible users in case the user is invisible himself
Merge pull request #1457 from Nik101010/fix-1456-feature

Forum/Thread: online users: invisible users < 2 not shown, refs #1456