fixed thrown TypeErrors on mac, while scrolling over the top edge

Due to the mac specific possibility to scroll above the top viewport edges (this nice viewport pull effect), you get some negative scroll and offset values. This causes a call of this.names[-1] which throws a TypeError. So we just test for a positive scroll value to prevent this.
Merge pull request #196 from sweaf/master

fixed thrown TypeErrors on mac, while scrolling over the top edge