avfilter/vf_ssim: remove unnecessary check

For the pointer have been checked in the previous few lines of code

Signed-off-by: Limin Wang <lance.lmwang@gmail.com>

libavfilter/vf_ssim.c

@@ -496,7 +496,7 @@ static int config_input_ref(AVFilterLink *inlink)
     if (!s->score)
         return AVERROR(ENOMEM);
 
-    for (int t = 0; t < s->nb_threads && s->score; t++) {
+    for (int t = 0; t < s->nb_threads; t++) {
         s->score[t] = av_calloc(s->nb_components, sizeof(*s->score[0]));
         if (!s->score[t])
             return AVERROR(ENOMEM);