cleanup: Don't savestatsandsettings if filesystem not init

This isn't necessary, but it does silence these annoying logs if you
pass an invalid argument or don't have data.zip:

    [ERROR] Could not get window size: Invalid renderer
    [WARN] Stats not loaded! Not writing unlock.vvv.
    [ERROR] Could not get window size: Invalid renderer
    [WARN] Settings not loaded! Not writing settings.vvv.

To do this, I've added FILESYSTEM_isInit().

desktop_version/src/main.cpp

@@ -697,7 +697,10 @@ int main(int argc, char *argv[])
 static void cleanup(void)
 {
     /* Order matters! */
-    game.savestatsandsettings();
+    if (FILESYSTEM_isInit()) /* not necessary but silences logs */
+    {
+        game.savestatsandsettings();
+    }
     gameScreen.destroy();
     graphics.grphx.destroy();
     graphics.destroy_buffers();