ip-neighbor: ip_neighbor_advertise() handles null

ip-neighbor: ip_neighbor_advertise() handles null

Type: fix
Fixes: cbe25aab3b

ip_neighbor_advertise() was calling one of both of
ip4_neighbor_advertise() and/or ip6_neighbor_advertise() with
&addr->{ip4|ip6} as an argument. If addr is null, which is likely
a requirement when the type is IP46_TYPE_BOTH, this results in a
SEGV.

Check addr and pass a pointer to one of it's members if it is not
null, otherwise pass null.

Change-Id: I6261bb8fe947365fe3d6c58788ea27d5cb28ff05
Signed-off-by: Matthew Smith <mgsmith@netgate.com>