Fix a directory traversal vuln

[zer0matt]

A Directory Traversal attack is made possible due to the lack of real control over the URL that is given as input.

Initializing the payload with this code: echo -e "GET /../../../../etc/passwd HTTP/1.0\r\nHost: localhost\r\n\r\n" | nc localhost 4000 gives the following output:

(Gasu16㉿VM)-[~/Tinyhttpd]
└─$ echo -e "GET /../../../../etc/passwd HTTP/1.0\r\nHost: localhost\r\n\r\n" | nc localhost 4000

HTTP/1.0 200 OK
Server: jdbhttpd/0.1.0
Content-Type: text/html

root:x:0:0:root:/root:/usr/bin/zsh
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
...

The patched code should be able to prevent this behavior and protect against path traversal attacks.