fileselector: Fixed regression of rELM3d2ae6d25ddc

https://phab.enlightenment.org/T198

After rELM3d2ae6d25ddc, buttons are destroyed when ok_cancel_set(true) is
called twice.
Spotted by uartie.

legacy/elementary/src/lib/elc_fileselector.c

@@ -1087,7 +1087,7 @@ _buttons_ok_cancel_set(Eo *obj, void *_pd, va_list *list)
 
         eo_do(obj, elm_wdg_theme(NULL));
      }
-   else
+   else if (!visible)
      {
         evas_object_del(sd->cancel_button);
         sd->cancel_button = NULL;