posix udp: More explicit checks for bogus address family

This triggered an error on FreeBSD because apparently FreeBSD will
return a different value when seeing an AF_UNIX socket with UDP.

src/platform/posix/posix_udp.c

@@ -215,9 +215,18 @@ nni_plat_udp_open(nni_plat_udp **upp, nni_sockaddr *bindaddr)
 	struct sockaddr_storage sa;
 	int                     rv;
 
-	if ((salen = nni_posix_nn2sockaddr(&sa, bindaddr)) < 1) {
+	if (bindaddr == NULL) {
 		return (NNG_EADDRINVAL);
 	}
+	switch (bindaddr->s_family) {
+	case NNG_AF_INET:
+	case NNG_AF_INET6:
+		break;
+	default:
+		return (NNG_EADDRINVAL);
+	}
+	salen = nni_posix_nn2sockaddr(&sa, bindaddr);
+	NNI_ASSERT(salen > 1);
 
 	// UDP opens can actually run synchronously.
 	if ((udp = NNI_ALLOC_STRUCT(udp)) == NULL) {

src/platform/udp_test.c

@@ -267,6 +267,7 @@ test_udp_bogus_bind(void)
 	// Some platforms reject IPC addresses altogether (Windows),
 	// whereas others just say it's not supported with UDP.
 	NUTS_ASSERT((rv == NNG_ENOTSUP) || (rv == NNG_EADDRINVAL));
+	NUTS_MSG("Result was %d %s", rv, nng_strerror(rv));
 
 	// NULL address also bad.
 	NUTS_FAIL(nng_udp_open(&u, NULL), NNG_EADDRINVAL);