gcoap: expire memo if there is nothing to wait for
gcoap: expire memo if there is nothing to wait for

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