Fix trac ticket #2242.
Note: under valid circumstances, when using -fix_sub_duration,
the last subtitle is a dummy termination packet, with no
allocated memory.
avcodec_free_frame(&input_streams[i]->decoded_frame);
av_dict_free(&input_streams[i]->opts);
free_buffer_pool(&input_streams[i]->buffer_pool);
+ avsubtitle_free(&input_streams[i]->prev_sub.subtitle);
avfilter_unref_bufferp(&input_streams[i]->sub2video.ref);
av_freep(&input_streams[i]->filters);
av_freep(&input_streams[i]);