asfdemux: Only use segment.start as seek_time if it is relative

asfdemux: Only use segment.start as seek_time if it is relative

Relying on the gst_segment_do_seek results for getting the
seeking time should not be used if seek type is SET because the
do_seek function makes the 'cur` position of the seek normalized
on the segment range. This means that seeking beyond the end of the
file might not result in instant EOS as it will end up seeking to
the final ts of the file.

In asf seeking to the final timestamp can lead to seeking to the last
keyframe, meaning a few seconds of video will still be played, which should
not be the desired behavior when seeking beyong file limits.