proto: fix logic inversion in previous commit

Rogue ! effectively disabled the ipv6 multicast check.  Fix cd089c52

Why is it always the simple changes that catch you out?

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

proto.c

@@ -131,7 +131,7 @@ parse_addr(struct interface *iface, const char *str, bool v6, int mask,
 	if (!parse_ip_and_netmask(af, str, &addr->addr, &addr->mask))
 		goto error;
 
-	if (!v6 && IN6_IS_ADDR_MULTICAST(&addr->addr.in6))
+	if (v6 && IN6_IS_ADDR_MULTICAST(&addr->addr.in6))
 		goto error;
 
 	if (broadcast)