vf_fps: set frame_rate.

(cherry picked from ffmpeg commit 0d249316)
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavfilter/vf_fps.c

@@ -116,9 +116,10 @@ static int config_props(AVFilterLink* link)
 {
     FPSContext   *s = link->src->priv;
 
-    link->time_base = (AVRational){ s->framerate.den, s->framerate.num };
-    link->w         = link->src->inputs[0]->w;
-    link->h         = link->src->inputs[0]->h;
+    link->time_base  = (AVRational){ s->framerate.den, s->framerate.num };
+    link->frame_rate = s->framerate;
+    link->w          = link->src->inputs[0]->w;
+    link->h          = link->src->inputs[0]->h;
 
     return 0;
 }