gluon-radvd: fix NULL deref

The specification says that the ifa_addr field of struct ifaddrs may be NULL,
and in Barrier Breaker, this does actually occur...

package/gluon-radvd/src/gluon-radvd.c

@@ -266,7 +266,7 @@ static void update_interface(void) {
 	memset(&G.iface.ifaddr, 0, sizeof(G.iface.ifaddr));
 
 	for (addr = addrs; addr; addr = addr->ifa_next) {
-		if (addr->ifa_addr->sa_family != AF_INET6)
+		if (!addr->ifa_addr || addr->ifa_addr->sa_family != AF_INET6)
 			continue;
 
 		const struct sockaddr_in6 *in6 = (const struct sockaddr_in6 *)addr->ifa_addr;