Fix out of bounds index access in getLabelMoment

Fix out of bounds index access in getLabelMoment

Previously, calling getLabelMoment with an out of bound index would cause an
error such as this:

```
Uncaught TypeError: Cannot read property 'null' of undefined
```

This happens because there is not always guaranteed to be a labelMoment on
at the current datasetIndex.

One example of this is practice comes from a this function call:

```js
// since the are not always guaranteed to be at least two labelMoments
//                                \ / this index can be out of bounds
//                                 |
var tickWidth = me.getPixelForTick(1) - me.getPixelForTick(0) - 6;
```

This patch simply ensures that the `labelMoments` for the `datasetIndex` are
defined before accessing properties on it.