bugfix: Do not try to apply SSP's base URL if REQUEST_URI does not match.

It is possible that the current script ($_SERVER['SCRIPT_FILENAME']) is inside SimpleSAMLphp's 'www' directory. However, even if that's the case, we should not enforce our base URL (as set in the 'baseurlpath' configuration option) if the request URI ($_SERVER['REQUEST_URI']) does not contain the relative path to the script. This is the case of AuthMemCookie, for example, where accessing a random URL protected by Apache, leads to the execution of a SimpleSAMLphp script, where SimpleSAML\Utils\HTTP::getSelfURL() must not try to be smart when guessing the current URL.