ArrayController#lookupItemController was called when `idx` was negative

I recently fixed out of range indexes (see #1983), but do not check negatives.
The reason is the same as #1982, [`objectAt` is called with `-1`](https://github.com/emberjs/ember.js/blob/master/packages/ember-runtime/lib/mixins/array.js#L380) when setting its content as an empty array.