Check for the 'member;range=0-1499' element before using it.

This is with Windows Server 2003, by the way, which (it seems) doesn't
always set this value. I haven't looked any deeper, but this change
does away with the error that I was getting:

ErrorException [ Notice ]: Undefined index: member;range=0-1499
DOCROOT\vendor\adldap\adldap\src\classes\adLDAPGroups.php [ 423 ]
Merge pull request #13 from samwilson/range

Check for the 'member;range=0-1499' element before using it.