vf_scale.c: propagate error code

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_scale.c

@@ -237,7 +237,7 @@ static int config_props(AVFilterLink *outlink)
     scale->isws[1] = sws_getContext(inlink ->w, inlink ->h/2, inlink ->format,
                                     outlink->w, outlink->h/2, outlink->format,
                                     scale->flags, NULL, NULL, NULL);
-    if (!scale->sws)
+    if (!scale->sws || !scale->isws[0] || !scale->isws[1])
         return AVERROR(EINVAL);
 
     if (inlink->sample_aspect_ratio.num){