ripngd: fix drop of multicast membership when the interface is down

ripngd: fix drop of multicast membership when the interface is down

When an interface is shut down, ripng_multicast_leave() is called after
ifp->flags is updated in ripng_interface_down(). So we shouldn't check
if the interface is up in order to proceed with the membership drop.

For consistency's sake, don't check for if_is_up() in
ripng_multicast_join() as well. In this case, this function is only
called when the interface is up, so the check was unnecessary.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>