WPS: Add workaround for missing Network Key attribute

Some deployed implementations do not include the mandatory Network
Key attribute when a WPS Credential is for an open network. Allow
this to improve interoperability since the actual key value is not
really needed for open networks.

src/wps/wps_attr_process.c

@@ -177,6 +177,13 @@ static int wps_process_cred_network_key(struct wps_credential *cred,
 	if (key == NULL) {
 		wpa_printf(MSG_DEBUG, "WPS: Credential did not include "
 			   "Network Key");
+		if (cred->auth_type == WPS_AUTH_OPEN &&
+		    cred->encr_type == WPS_ENCR_NONE) {
+			wpa_printf(MSG_DEBUG, "WPS: Workaround - Allow "
+				   "missing mandatory Network Key attribute "
+				   "for open network");
+			return 0;
+		}
 		return -1;
 	}