fix directory traversal issue

[d3zorg]

This PR fixes directory traversal issue, example of attack on demo server:

$ curl -v --path-as-is http://134.209.196.181:8080/../../../../../../../../../../../../etc/passwd

• Trying 134.209.196.181...
• TCP_NODELAY set
• Connected to 134.209.196.181 (134.209.196.181) port 8080 (#0)

  GET /../../../../../../../../../../../../etc/passwd HTTP/1.1 Host: 134.209.196.181:8080 User-Agent: curl/7.64.1 Accept: /

  < HTTP/1.1 200 OK < Content-Type: text/plain < Content-Length: 1222 < Last-Modified: Wed, 10 Feb 2021 08:50:42 GMT < Expires: Wed, 17 Feb 2021 08:50:42 GMT < Date: Fri, 23 Jul 2021 19:04:55 GMT < Connection: keep-alive < Keep-Alive: timeout=5 < root:x:0:0:root:/root:/bin/ash bin:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/sbin:/sbin/nologin adm:x:3:4:adm:/var/adm:/sbin/nologin lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin sync:x:5:0:sync:/sbin:/bin/sync shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown halt:x:7:0:halt:/sbin:/sbin/halt mail:x:8:12:mail:/var/mail:/sbin/nologin news:x:9:13:news:/usr/lib/news:/sbin/nologin uucp:x:10:14:uucp:/var/spool/uucppublic:/sbin/nologin operator:x:11:0:operator:/root:/sbin/nologin man:x:13:15:man:/usr/man:/sbin/nologin postmaster:x:14:12:postmaster:/var/mail:/sbin/nologin cron:x:16:16:cron:/var/spool/cron:/sbin/nologin ftp:x:21:21::/var/lib/ftp:/sbin/nologin sshd:x:22:22:sshd:/dev/null:/sbin/nologin at:x:25:25:at:/var/spool/cron/atjobs:/sbin/nologin squid:x:31:31:Squid:/var/cache/squid:/sbin/nologin xfs:x:33:33:X Font Server:/etc/X11/fs:/sbin/nologin games:x:35:35:games:/usr/games:/sbin/nologin cyrus:x:85:12::/usr/cyrus:/sbin/nologin vpopmail:x:89:89::/var/vpopmail:/sbin/nologin ntp:x:123:123:NTP:/var/empty:/sbin/nologin smmsp:x:209:209:smmsp:/var/spool/mqueue:/sbin/nologin guest:x:405:100:guest:/dev/null:/sbin/nologin nobody:x:65534:65534:nobody:/:/sbin/nologin node:x:1000:1000:Linux User,,,:/home/node:/bin/sh

• Connection #0 to host 134.209.196.181 left intact
• Closing connection 0