Nav highlight does not work on first nav items

[robhrt7]

In cases when there is not much content before first section, highlight of first sections in inner navigation is not working after click.

Steps to reproduce:

1. Go to http://sourcejs.com/docs/base/
2. Click to inner navigation section 3.
3. Click to inner navigation section 1.
4. First nav item is not highlighted and URL is not updated.

@fromanywhere

[robhrt7]

Fixed by changing safe scrolling coordinates. Might need some deep refactoring later, if first section position will get closer to the top of the page.