netlink: fix accessing freed memory

The check for if_addrlen in dump_iface() is not sufficient to determine
if we still have a valid if_addr. Rather than directly accessing if_addr
check the STAILQ (for the first entry).
This avoids panics when destroying cloned interfaces as experienced with
net80211 wlan ones.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: jhibbits (earlier version), kp
Differential Revision: https://reviews.freebsd.org/D42027