[mod_alias] security: potential path traversal with specific configs

Security: potential path traversal of a single directory above the alias
target with a specific mod_alias config where the alias which is matched
does not end in '/', but alias target filesystem path does end in '/'.

e.g. server.docroot = "/srv/www/host/HOSTNAME/docroot"
     alias.url = ( "/img" => "/srv/www/hosts/HOSTNAME/images/" )

If a malicious URL "/img../" were passed, the request would be
for directory "/srv/www/hosts/HOSTNAME/images/../" which would resolve
to "/srv/www/hosts/HOSTNAME/".  If mod_dirlisting were enabled, which
is not the default, this would result in listing the contents of the
directory above the alias.  An attacker might also try to directly
access files anywhere under that path, which is one level above the
intended aliased path.

credit: Orange Tsai(@orange_8361) from DEVCORE