WPS: Check wps_build_wfa_ext() return value consistently (CID 68104)

While this call cannot really fail, check the return value to be more
consistent with all the other wps_build_wfa_ext() calls.

Signed-off-by: Jouni Malinen <j@w1.fi>

src/wps/wps_upnp.c

@@ -596,7 +596,10 @@ static struct wpabuf * build_fake_wsc_ack(void)
 	wpabuf_put_be16(msg, ATTR_REGISTRAR_NONCE);
 	wpabuf_put_be16(msg, WPS_NONCE_LEN);
 	wpabuf_put(msg, WPS_NONCE_LEN);
-	wps_build_wfa_ext(msg, 0, NULL, 0);
+	if (wps_build_wfa_ext(msg, 0, NULL, 0)) {
+		wpabuf_free(msg);
+		return NULL;
+	}
 	return msg;
 }