net_udp: set IPV6_MULTICAST_IF only of not default

macOS fails on it otherwise (at least 15.0), although it shouldn't
according to man ip6(4) (that is, however, presumably inherited from BSD,
because FreeBSD has the same).

src/rtp/net_udp.c

@@ -507,10 +507,13 @@ static bool udp_join_mcast_grp6(struct in6_addr sin6_addr, int rx_fd, int tx_fd,
                 } else {
                         MSG(WARNING, "Using multicast but not setting TTL.\n");
                 }
-                if (SETSOCKOPT(tx_fd, IPPROTO_IPV6, IPV6_MULTICAST_IF,
-                                        (char *)&ifindex, sizeof(ifindex)) != 0) {
-                        socket_error("setsockopt IPV6_MULTICAST_IF");
-                        return false;
+                if (ifindex != 0) {
+                        if (SETSOCKOPT(tx_fd, IPPROTO_IPV6, IPV6_MULTICAST_IF,
+                                       (char *) &ifindex,
+                                       sizeof(ifindex)) != 0) {
+                                socket_error("setsockopt IPV6_MULTICAST_IF");
+                                return false;
+                        }
                 }
         }