This fixes at least two crashes reported through the crashreporter:
http://crashreport.libreoffice.org/stats/signature/avmedia::MediaItem::getTime()
http://crashreport.libreoffice.org/stats/signature/avmedia::MediaItem::getDuration()

Change-Id: I92eeca229fa46921317586d0317e9f00309e793b
Reviewed-on: https://gerrit.libreoffice.org/34710Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>