Error thrown on category scale when elements are missing

[tiagoroldao]

The latest change to core.controller requests !meta.data[found._index]._view.skip without checking for meta.data[found._index]'s existence, in line 417, causing an error to throw if all data arrays aren't of the same length.

Hover over the last pair of bars here to see error in console:
http://plnkr.co/edit/TEjG1eVa8z5odfPAq5n3?p=preview

This is nonexistent in 2.1.6.

[The reason I found this is due to seeking to create category controllers that can use "coordinates" (e.g. {x: "labelName", y: value} (WIP, obviously).]

[tiagoroldao]

Fix is very simple, have a branch ready for PR.

[panzarino]

Please submit a PR if you have a fix