Fixes issue for me when doing transcode{}:gather: --sout-keep and subtitle
overlaying from dvbs. Otherwise keeps last subtitles on video if new channel
doesn't contain dvbs track.

also consistenty as spu_Create is called on transcode_spu_new()
(cherry picked from commit 41b8c2e1f660c767ee7a96ab69204b94b1470ca8)
Signed-off-by: Rémi Duraffort <ivoire@videolan.org>