lavfi/vf_fps: check flow before sending more frames

Analyzed by Paul B Mahol <onemda@gmail.com>.

Fixes OOM in #9081.

libavfilter/vf_fps.c

@@ -351,7 +351,7 @@ static int activate(AVFilterContext *ctx)
     if (s->frames_count > 0) {
         ret = write_frame(ctx, s, outlink, &again);
         /* Couldn't generate a frame, so schedule us to perform another step */
-        if (again)
+        if (again && ff_inoutlink_check_flow(inlink, outlink))
             ff_filter_set_ready(ctx, 100);
         return ret;
     }