avfilter/vf_subtitles: Fix leaks on failure

avfilter/vf_subtitles: Fix leaks on failure

init_subtitles() sometimes returned directly upon error without cleaning
up after itself. The easiest way to trigger this is by using
picture-based subtitles; it is also possible to run into this in case of
missing decoders or allocation failures.

Furthermore, return the proper error code in case of missing decoder.

Reviewed-by: Nicolas George <george@nsup.org>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>