ffmpeg: free empty subtitles as well in transcode_subtitles

Even if it does not matter at the moment, because subtitles with num_rect == 0
have no memory allocated, this is how we expect the users to use the API, a
returned AVSubtitle should be freed with avsubtitle_free.

Signed-off-by: Marton Balint <cus@passwd.hu>
Reviewed-by: Nicolas George <george@nsup.org>