Fix XSS problem, regression in the 3.4 branch.

Dev releases until -beta2 are vulnerable.
Thanks to Aung Khant from YGN Ethical Hacker Group (http://yehg.net/) for reporting this issue.

libraries/header.inc.php

@@ -121,7 +121,7 @@ if (!$GLOBALS['is_ajax_request']) {
                     printf($item,
                             $GLOBALS['cfg']['DefaultTabDatabase'],
                             PMA_generate_common_url($GLOBALS['db']),
-                            $GLOBALS['db'],
+                            htmlspecialchars($GLOBALS['db']),
                             __('Database'),
                             's_tbl.png');
                     // if the table is being dropped, $_REQUEST['purge'] is set