lavfi/vf_vmafmotion: use av_err2str to simplify code

No need to explicitly specify the buffer here as it is only
ever passed to av_log, so av_err2str can be used.

libavfilter/vf_vmafmotion.c

@@ -318,10 +318,8 @@ static av_cold int init(AVFilterContext *ctx)
             s->stats_file = avpriv_fopen_utf8(s->stats_file_str, "w");
             if (!s->stats_file) {
                 int err = AVERROR(errno);
-                char buf[128];
-                av_strerror(err, buf, sizeof(buf));
                 av_log(ctx, AV_LOG_ERROR, "Could not open stats file %s: %s\n",
-                       s->stats_file_str, buf);
+                       s->stats_file_str, av_err2str(err));
                 return err;
             }
         }