avfilter/vsrc_sierpinski: set frame_rate to outlink

libavfilter/vsrc_sierpinski.c

@@ -149,6 +149,7 @@ static int config_output(AVFilterLink *outlink)
     outlink->h = s->h;
     outlink->time_base = av_inv_q(s->frame_rate);
     outlink->sample_aspect_ratio = (AVRational) {1, 1};
+    outlink->frame_rate = s->frame_rate;
     if (s->seed == -1)
         s->seed = av_get_random_seed();
     av_lfg_init(&s->lfg, s->seed);