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>

ffmpeg.c

@@ -1828,7 +1828,7 @@ static int transcode_subtitles(InputStream *ist, AVPacket *pkt, int *got_output)
     sub2video_update(ist, &subtitle);
 
     if (!subtitle.num_rects)
-        return ret;
+        goto out;
 
     for (i = 0; i < nb_output_streams; i++) {
         OutputStream *ost = output_streams[i];
@@ -1839,6 +1839,7 @@ static int transcode_subtitles(InputStream *ist, AVPacket *pkt, int *got_output)
         do_subtitle_out(output_files[ost->file_index]->ctx, ost, ist, &subtitle);
     }
 
+out:
     avsubtitle_free(&subtitle);
     return ret;
 }