Do not process subforms with PMA_MINIMUM_COMMON

In such case needed infrastructure is not loaded, so related code won't
work anyway.

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

libraries/common.inc.php

@@ -231,7 +231,7 @@
  * so we now check if a subform is submitted
  */
 $__redirect = null;
-if (isset($_POST['usesubform'])) {
+if (isset($_POST['usesubform']) && ! defined('PMA_MINIMUM_COMMON')) {
     // if a subform is present and should be used
     // the rest of the form is deprecated
     $subform_id = key($_POST['usesubform']);