Fix an s_server arbitrary file read issue on Windows

Running s_server in WWW mode on Windows can allow a client to read files
outside the s_server directory by including backslashes in the name, e.g.

GET /..\myfile.txt HTTP/1.0

There exists a check for this for Unix paths but it is not sufficient
for Windows.

Since s_server is a test tool no CVE is assigned.

Thanks to Jobert Abma for reporting this.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10215)
