Fixed error when no snappable objects where found

I use this plugin on my site in desktop mode, but I want to disable it in tablet/phone layout. This is done by adding and removing the class "snap" from my sections. This works alright (is there any other way to turn of snapping temporarily?). But then I get a javascript error:

```
Uncaught TypeError: Cannot read property 'top' of undefined jquery.panelSnap.js:305
```