ffmpeg: free last sub when using -fix_sub_duration.

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.

ffmpeg.c

@@ -459,6 +459,7 @@ static void exit_program(void)
         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]);