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/FileSystemUtils.h

@@ -10,6 +10,7 @@ class binaryBlob;
 namespace tinyxml2 { class XMLDocument; }
 
 int FILESYSTEM_init(char *argvZero, char* baseDir, char* assetsPath);
+bool FILESYSTEM_isInit(void);
 void FILESYSTEM_deinit(void);
 
 char *FILESYSTEM_getUserSaveDirectory(void);