wrap: prevent add_file from writing empty clusters

src/wrap.c

@@ -66,6 +66,10 @@ static enum ec3_status add_file(
 
 	while (1) {
 		size_t r = fread(buf, 1, cluster_size, inp);
+		if (r == 0) {
+			break;
+		}
+
 		size_t w;
 		status = ec3_tag_ioctx_write_cluster(tag, i++, buf, r, &w);