ndp: cope with unresolved neighbours

If we've not (yet) resolved a neighbour nda_lladdr will be NULL, and
NLA_DATA_LEN(neigh->nda_lladdr) will dereference a NULL pointer.

Avoid that by checking nda_lladdr first, and only dereferencing if it's
not NULL.

Test case:
	ping6 -c 1 <non-existant neighbour>
	ndp -a

Reviewed by:	melifaro
MFC after:	3 days
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D41903

usr.sbin/ndp/ndp_netlink.c

@@ -230,9 +230,12 @@ print_entry(struct snl_parsed_neigh *neigh, struct snl_parsed_link_simple *link)
 		.sdl_family = AF_LINK,
 		.sdl_type = link->ifi_type,
 		.sdl_len = sizeof(struct sockaddr_dl),
-		.sdl_alen = NLA_DATA_LEN(neigh->nda_lladdr),
 	};
-	memcpy(sdl.sdl_data, NLA_DATA(neigh->nda_lladdr), sdl.sdl_alen);
+
+	if (neigh->nda_lladdr) {
+		sdl.sdl_alen = NLA_DATA_LEN(neigh->nda_lladdr),
+		memcpy(sdl.sdl_data, NLA_DATA(neigh->nda_lladdr), sdl.sdl_alen);
+	}
 
 	addrwidth = strlen(host_buf);
 	if (addrwidth < W_ADDR)