Correctly indicate no auth result in AAR

It is not valid to have no authentication results; if no message
authentication was performed the special value no-result
(`[CFWS] ";" [CFWS] "none"`) should be used.

trusteddomainproject#174

Author: flowerysong

libopenarc/arc.c

@@ -3272,8 +3272,12 @@ arc_getseal(ARC_MESSAGE *msg, ARC_HDRFIELD **seal, char *authservid,
 	arc_dstring_printf(dstr, "ARC-Authentication-Results:i=%u; %s",
 	                   msg->arc_nsets + 1,
 	                   msg->arc_authservid);
-	if (ar != NULL)
+	if (ar == NULL) {
+		/* no-result per RFC 8601 2.2 */
+		arc_dstring_printf(dstr, "; none");
+	} else {
 		arc_dstring_printf(dstr, "; %s", (char *) ar);
+	}
 
 	status = arc_parse_header_field(msg, arc_dstring_get(dstr),
 	                                arc_dstring_len(dstr), &h);