lavfi/scale: return error code in case of failed reconfiguration in start_frame()

Avoid an assert, since now the function supports error code return.

libavfilter/vf_scale.c

@@ -316,10 +316,9 @@ static int start_frame(AVFilterLink *link, AVFilterBufferRef *picref)
         link->dst->inputs[0]->h      = picref->video->h;
 
         if ((ret = config_props(outlink)) < 0)
-            av_assert0(0); //what to do here ?
+            return ret;
     }
 
-
     if (!scale->sws) {
         outpicref = avfilter_ref_buffer(picref, ~0);
         if (!outpicref)