Does this fix it?

> diff --git a/fftools/ffmpeg_dec.c b/fftools/ffmpeg_dec.c
> index 5020684a28..5dca26fbb2 100644
> --- a/fftools/ffmpeg_dec.c
> +++ b/fftools/ffmpeg_dec.c
> @@ -1621,7 +1621,11 @@ static int dec_open(DecoderPriv *dp, AVDictionary **dec_opts,
>              dp->dec_ctx->extra_hw_frames = extra_frames;
>      }
>  
> -    dp->dec.subtitle_header      = dp->dec_ctx->subtitle_header;
> +    if (dp->dec_ctx->subtitle_header) {
> +        dp->dec.subtitle_header) = av_memdup(dp->dec_ctx->subtitle_header, dp->dec_ctx->subtitle_header_size);
> +        if (!dp->dec.subtitle_header)
> +            return AVERROR(ENOMEM);
> +    }
>      dp->dec.subtitle_header_size = dp->dec_ctx->subtitle_header_size;
>  
>      if (param_out) {