CRM-8599 Untested var causes a non-blocking error when disabling wordreplacements

Configure a set of word replacement.
Disable them and validate, a notice appears :
"""
Notice : Undefined index: cb dans CRM_Admin_Form_WordReplacements->postProcess() (ligne 209 dans /var/www/drupal/sites/all/modules/civicrm/CRM/Admin/Form/WordReplacements.php).
"""

In /var/www/drupal/sites/all/modules/civicrm/CRM/Admin/Form/WordReplacements.php, the is_array function is used on the $params['cb'] var, but it seems that in this case $params doesn't provide the cb key, the following patch adds a little isset test in the if block.