COre/Mail: Don't allow to mark delete mails as read.

src/server/game/Handlers/MailHandler.cpp

@@ -345,7 +345,7 @@ void WorldSession::HandleMailMarkAsRead(WorldPacket& recvData)
 
     Player* player = _player;
     Mail* m = player->GetMail(mailId);
-    if (m)
+    if (m && m->state != MAIL_STATE_DELETED)
     {
         if (player->unReadMails)
             --player->unReadMails;