Only check permissions if core permissions return false

In my original code there was a bug that I just fixed for 6.x. http://drupalcode.org/project/contact_permissions.git/commitdiff/48f3022

from 6.x code:

$access = _contact_user_tab_access($account);
-  if ($access) {
+  if (!$access) {

We should port this to 7.x