Prevent NPE during ion-slide-box drag

```
Uncaught TypeError: Cannot read property 'contains' of undefined ionic.bundle.js:6182
ionic.views.ListView.ionic.views.View.inherit._getItem ionic.bundle.js:6182
ionic.views.ListView.ionic.views.View.inherit._startDrag ionic.bundle.js:6223
ionic.views.ListView.ionic.views.View.inherit._handleDrag ionic.bundle.js:6266
(anonymous function) ionic.bundle.js:6088
triggerEvent ionic.bundle.js:769
dragGesture ionic.bundle.js:1804
detect ionic.bundle.js:1347
bindDomOnTouch ionic.bundle.js:896
```