WPS UPnP: Check os_get_random() result more consistently

While the value here would not matter much, make the implementation more
consistent (CID 72692).

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

src/wps/wps_upnp_ssdp.c

@@ -317,7 +317,8 @@ static void advertisement_state_machine_handler(void *eloop_data,
 			 * (see notes above)
 			 */
 			next_timeout_msec = 0;
-			os_get_random((void *) &r, sizeof(r));
+			if (os_get_random((void *) &r, sizeof(r)) < 0)
+				r = 32768;
 			next_timeout_sec = UPNP_CACHE_SEC / 4 +
 				(((UPNP_CACHE_SEC / 4) * r) >> 16);
 			sm->advertise_count++;