Truncate only long passwords

This avoids problems with certain PHP versions returning false when
first parameter to substr is ''.

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

libraries/common.inc.php

@@ -868,7 +868,7 @@
                 . ' ' . $cfg['Server']['auth_type']
             );
         }
-        if (isset($_REQUEST['pma_password'])) {
+        if (isset($_REQUEST['pma_password']) && strlen($_REQUEST['pma_password']) > 256) {
             $_REQUEST['pma_password'] = substr($_REQUEST['pma_password'], 0, 256);
         }
         include_once  './libraries/plugins/auth/' . $auth_class . '.class.php';