Check length of the scrolledSections array before taking first element.
Check length of the scrolledSections array before taking first element.

In the previous version there was check if first element defined or not.  @alvarotrigo noted (https://github.com/alvarotrigo/fullPage.js/pull/731#issuecomment-56633395) that checking array length looks like better solution and I agree :).

fixes #731, #752

publicJorn/fullPage.js,publicJorn/fullPage.js