avfilter/vf_libplacebo: don't explode on zero FPS information

While technically not specifiad as valid by the AVFilterLink documentation,
it is currently possible to get an FPS of zero from various sources inside
libavfilter (notably vf_buffersrc).

Avoid a division by zero and resulting infinity when this happens.

Author: haasn

libavfilter/vf_libplacebo.c

@@ -1073,7 +1073,7 @@ static int libplacebo_activate(AVFilterContext *ctx)
             in->qstatus = pl_queue_update(in->queue, &in->mix, pl_queue_params(
                 .pts            = out_pts * av_q2d(outlink->time_base),
                 .radius         = pl_frame_mix_radius(&s->opts->params),
-                .vsync_duration = av_q2d(av_inv_q(l->frame_rate)),
+                .vsync_duration = l->frame_rate.num ? av_q2d(av_inv_q(l->frame_rate)) : 0,
             ));
 
             switch (in->qstatus) {