Disable setup if configuration already exists

The setup allows to figure out quite a lot about system and network, so
it's safer to block access to it once phpMyAdmin has been configured.

Signed-off-by: Michal Čihař <michal@cihar.com>

setup/index.php

@@ -12,6 +12,10 @@
  */
 require './lib/common.inc.php';
 
+if (file_exists(CONFIG_FILE)) {
+    PMA_fatalError(__('Configuration already exists, setup is disabled!'));
+}
+
 $page = PMA_isValid($_GET['page'], 'scalar') ? $_GET['page'] : null;
 $page = preg_replace('/[^a-z]/', '', $page);
 if ($page === '') {