Fixed a crash in libvlc if there are no subtitles available

kaffeine crashes, when watching DVB and there are no subtitles for the
current channel available.
DvbLiveView::pmtSectionChanged sets subtitlePid to -1 if there are no
subtitles available. VlcMediaWidget::setCurrentSubtitle unconditionally
adds 1 to the requested subtitle index.
Now we're trying to activate subtitle 0 which does not exist.
If subtitle -1 was given then we really want to disable subtitles
(= sending -1 to libvlc).

This patch also makes setCurrentSubtitle more robust.

REVIEW: 106297