用 NULL 判断是否取到 ERU_INFO

c

    if (clcf->server_tokens) {
        u_char *other_info = (u_char *)getenv("ERU_INFO");
        if (other_info == NULL) {
            other_info = (u_char *) "null_eru";
        }
        u_char eru_tail[48];
        // ngx_snprintf 会把 eru_tail 后面的空位也返回来，然后就乱码 
        ngx_sprintf(eru_tail, "<hr><center>%s</center>\r\n</body>\r\n</html>\r\n", other_info);
        len = sizeof(eru_tail) - 1;
        tail = eru_tail;
    } else {
        len = sizeof(ngx_http_error_tail) - 1;
        tail = ngx_http_error_tail;
    }