[ticket/11355] Wrong error message when no user is selected.

In ACP group membership management, if you select 'remove member
from group' without selecting any users and submit,it will display
a wrong error after confirmbox. Since no users are selected, this
should not go even until confirmbox. Added a new condition to check
weather any users are selected when the action is 'deleteusers'
in acp_groups.php and disply a correct error.

PHPBB3-11355
[ticket/11355] Referred proper variable when validating selection.

Earlier PR was referring wrong variable $name_ary. Changed it to
$mark_ary which actually contains selected user ids.

PHPBB3-11355
Merge branch 'ticket/11355' into develop-olympus

* ticket/11355:
  [ticket/11355] Referred proper variable when validating selection.
  [ticket/11355] Wrong error message when no user is selected.