fix android JSON.parse(null) bug

bug fix for Android JSON.parse bug

## Error message


```
Uncaught illegal access at file:///android_asset/www/plugins/memory.enchant.js:667
```

## See also

See also for same bug

Android + JSON.parse(NULL) = Uncaught illegal access · Issue #48 · brianleroux/lawnchair
https://github.com/brianleroux/lawnchair/issues/48

In Japanese
http://metamosoft.hatenablog.com/entry/2013/06/16/143156?_ga=1.145227508.159
6586333.1367849780