avfilter/vf_zscale: fix number of threads

Make sure it is between [1, MAX_THERADS] and also take into account the outlink
size in order not to request zero height output from zscale.

Signed-off-by: Marton Balint <cus@passwd.hu>

libavfilter/vf_zscale.c

@@ -799,7 +799,7 @@ static int filter_frame(AVFilterLink *link, AVFrame *in)
         link->dst->inputs[0]->w      = in->width;
         link->dst->inputs[0]->h      = in->height;
 
-        s->nb_threads = FFMIN(ff_filter_get_nb_threads(ctx), link->h / 8);
+        s->nb_threads = av_clip(FFMIN(ff_filter_get_nb_threads(ctx), FFMIN(link->h, outlink->h) / 8), 1, MAX_THREADS);
         s->in_colorspace = in->colorspace;
         s->in_trc = in->color_trc;
         s->in_primaries = in->color_primaries;