VLCMovieViewController: Fix seeking in live streams
Live streams have sometimes no valid duration, so setting the
_positionSet state only in case of _mediaDuration > 0 leads to
an issue that seeking in such streams only works for the first time,
as _positionSet is initialized as NO, then it is set to YES in
_setPositionForReal and would be never set back to NO, causing all
future seeks to be ignored.