Make sure the interface still exists before updating it.

Make sure the interface still exists before updating it.

When we get a route message, for example an address being added
(RTM_NEWADDR, but the problem exists with most of the route messages)
and the interface gets unplugged at just the right moment
if_nametoindex(3) will return NULL. We will pass NULL through
update_iface() to get_xflags() which will then crash because we
dereference the NULL pointer there.

OK kn