ipv6/nib: bugfix of 6CO length checking
ipv6/nib: bugfix of 6CO length checking

miri64/RIOT,ant9000/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kYc0o/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kaspar030/RIOT,miri64/RIOT,jasonatran/RIOT,miri64/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,kYc0o/RIOT,ant9000/RIOT,RIOT-OS/RIOT,miri64/RIOT,OlegHahm/RIOT,kaspar030/RIOT,OlegHahm/RIOT,jasonatran/RIOT,kYc0o/RIOT,jasonatran/RIOT,ant9000/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,miri64/RIOT,kaspar030/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kaspar030/RIOT