netlink: Avoid undefined behaviour

Even though it is not dereferenced, it is UB to take the address of an
out of bounds array element.

Reviewed by: pouria, bz, des, adrian
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D57158

sys/netlink/netlink_generic.c

@@ -127,13 +127,13 @@ genl_handle_message(struct nlmsghdr *hdr, struct nl_pstate *npt)
 	}
 
 	family_id = hdr->nlmsg_type - GENL_MIN_ID;
-	gf = &families[family_id];
 	if (__predict_false(family_id >= MAX_FAMILIES ||
-	    gf->family_name == NULL)) {
+	    families[family_id].family_name == NULL)) {
 		NLP_LOG(LOG_DEBUG, nlp, "invalid message type: %d",
 		    hdr->nlmsg_type);
 		return (ENOTSUP);
 	}
+	gf = &families[family_id];
 
 	struct genlmsghdr *ghdr = (struct genlmsghdr *)(hdr + 1);