Handle multiple zero-offset Scrollspy elements.

Handle multiple zero-offset Scrollspy elements.

When the first two elements in a scrollspy content block have a document
offset of zero (i.e. they're hard against the top of the page),
Scrollspy would switch between them on every scroll event.

This could happen, for example, in a system of nested sections:

```
<section id="animals">
  <section id="dogs">
	Content
  </section>
</section>
```

This ocurred because Scrollspy's check to see if it's at the end of the
array of sections uses `!arr[index]`. This misses the case where
`arr[index]` does exist and is zero.

This commit explicitly checks the array bounds.