In some cases, demux_mkv will detect a start time slightly above 0, but
there might still be a subtitle starting at exactly 0. When the player
rebases the timestamps to assumed start time, the subtitle will have a
slightly negative timestamp in the end. libavcodec's subtitle converter
turns this into a larger number due to underflow. Fix by clamping
subtitles always to 0, which may or may not be what you want.
At least it fixes#5047.
c23c9e22ae