[bug/58755] Fix a redirection bug that can occur after login

This issue affects any forum (i.e. https://myforum/phpBB3/) where:
 - the forum is located in a directory underneath the web root (i.e., NOT https://myforum/)
 - a user accesses the forum with a URI pointing to a directory rather than a script (i.e., NOT https://myforum/phpBB3/index.php)
 - the URI used ends in a slash (i.e., NOT https://myforum/phpBB3)

If these conditions are met, after successful login the user is redirected to an invalid URI (i.e., https://myforum/phpBB3/phpBB3?sid=).

This change fixes extract_current_page() to handle the case correctly where the URI ends in a slash and is not the web root. So after successful login, the redirection back to the main page will work (i.e., https://myforum/phpBB3/?sid=)

phpBB/includes/session.php

@@ -83,7 +83,7 @@ function extract_current_page($root_path)
 		$query_string = trim(implode('&', $use_args));
 
 		// basenamed page name (for example: index.php)
-		$page_name = basename($script_name);
+		$page_name = (substr($script_name, -1, 1) == '/') ? '' : basename($script_name);
 		$page_name = urlencode(htmlspecialchars($page_name));
 
 		// current directory within the phpBB root (for example: adm)