Avoid crash after driver init failure

hostapd_flush_old_stations() needs to check whether the driver
initialization was successful since it gets called on an error path.

src/ap/hostapd.c

@@ -299,7 +299,7 @@ static int hostapd_flush_old_stations(struct hostapd_data *hapd)
 {
 	int ret = 0;
 
-	if (hostapd_drv_none(hapd))
+	if (hostapd_drv_none(hapd) || hapd->drv_priv == NULL)
 		return 0;
 
 	wpa_printf(MSG_DEBUG, "Flushing old station entries");