Fix incorrect check whether the active user is the author

Author: BurntimeX

files/lib/data/conversation/ConversationAction.class.php

@@ -924,7 +924,7 @@ public function getAddParticipantsForm()
             'excludedSearchValues' => $this->conversation->getParticipantNames(
                 false,
                 true,
-                $this->conversation == WCF::getUser()->userID
+                $this->conversation->userID == WCF::getUser()->userID
             ),
             'maxItems' => WCF::getSession()->getPermission('user.conversation.maxParticipants') - $this->conversation->participants,
             'canAddGroupParticipants' => WCF::getSession()->getPermission('user.conversation.canAddGroupParticipants'),