dashdemux: corrected computation of period's duration

dashdemux: corrected computation of period's duration

According to ISO/IEC 23009-1:2014(E), chapter 5.3.2.1
"The Period extends until the PeriodStart of the next Period, or until
the end of the Media Presentation in the case of the last Period."

This means that a configured value for optional attribute period duration
should be ignored if the next period contains a start attribute or it is
the last period and the MPD contains a mediaPresentationDuration attribute.

https://bugzilla.gnome.org/show_bug.cgi?id=750797