The already parsed subtitles (contained in an FFDemuxSubtitlesQueue)
would leak if an error happened upon reading a subsequent subtitle
or if creating the extradata failed.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit 5ab39c2d8c1e5e00b48d758eee7d5ae435a99ef7)