Fix a strange bug where hammer.js was confused between drag and transform

With following settings:
````
drag_min_distance: 0,
scale_threshold: 0,
prevent_default: true
```
I had the following problem on the change from drag to transform (i.e. one starts to drag with one finger and later a second finger is recognized so the mode changes to transform):

```TypeError: 'undefined' is not an object (evaluating '_pos.startCenter.x')```

It seemed that ```_first``` was already set to ```true``` but ```_pos.startCenter``` was not defined yet. 

I'm not sure if this is the right way to fix this, but It did the trick for me.