When there are gaps between segments in the DASH manifest (e.g. it's a
recording of a live stream during which the camera was turned on and off
a couple times), seeking to a timestamp that belongs into a gap leads to
repeat_index calculation yielding a nonsensical negative number (because
ts < segment->start).
In such event set the playback to continue at the first repetition of
the segment that follows after the gap.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8620>
ddd6a708cb