bgpd: don't try to reconcile AS4_PATH with NULL

bgpd: don't try to reconcile AS4_PATH with NULL

bgp_attr_munge_as4_attrs would previously try to reintegrate an AS4_PATH
with a NULL AS_PATH, leading to a rather nasty SEGV.  Let's go by
RFC6793 and treat missing AS_PATH as 0-length AS_PATH, which in turn
means discarding the AS4_PATH.

[NB: we don't actually stick to the actual rule, which is discarding
AS4_PATH if it's longer than AS_PATH; indeed we should probably fix that
too]

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>