Avoid passing a NULL uid to sprintf(), as it annoys OpenBSD.

html.c

@@ -4586,7 +4586,7 @@ int html_get_handler(const xs_dict *req, const char *q_path,
     if (!uid || !user_open(&snac, uid)) {
         /* invalid user */
         status = grave(uid, 0) ? HTTP_STATUS_GONE : HTTP_STATUS_NOT_FOUND;
-        srv_debug(1, xs_fmt("html_get_handler bad user %s %d", uid, status));
+        srv_debug(1, xs_fmt("html_get_handler bad user %s %d", xs_or(uid, "(null)"), status));
         return status;
     }