lavf/concat: Never fail for sample aspect ratio 0:1.

Fixes ticket #2456.

Reviewed-by: Nicolas George

libavfilter/avf_concat.c

@@ -134,10 +134,13 @@ static int config_output(AVFilterLink *outlink)
     outlink->format              = inlink->format;
     for (seg = 1; seg < cat->nb_segments; seg++) {
         inlink = ctx->inputs[in_no += ctx->nb_outputs];
+        if (!outlink->sample_aspect_ratio.num)
+            outlink->sample_aspect_ratio = inlink->sample_aspect_ratio;
         /* possible enhancement: unsafe mode, do not check */
         if (outlink->w                       != inlink->w                       ||
             outlink->h                       != inlink->h                       ||
-            outlink->sample_aspect_ratio.num != inlink->sample_aspect_ratio.num ||
+            outlink->sample_aspect_ratio.num != inlink->sample_aspect_ratio.num &&
+                                                inlink->sample_aspect_ratio.num ||
             outlink->sample_aspect_ratio.den != inlink->sample_aspect_ratio.den) {
             av_log(ctx, AV_LOG_ERROR, "Input link %s parameters "
                    "(size %dx%d, SAR %d:%d) do not match the corresponding "