Workaround for VideoTag#buffered Bug in Firefox

For some video files the video tag of Firefox 32 reports negative time
stamps in buffered ranges. The maximum buffered time stamp was
therefore always reported as 0, causing constant buffer underrun
waiting.

Use difference of time stamps as end time stamp in this case.
Merge pull request #116 from tf/video-negative-buffered

Workaround for VideoTag#buffered Bug in Firefox