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
Merge pull request #771 from tkirill/master

Check length of the scrolledSections array before taking first element.