CRM-14356 - CRM_Core_Session - Handle Drupal's lazy sessions

This is Ken's original patch from
http://forum.civicrm.org/index.php/topic,31800.msg136249.html
CRM-14356 - CRM_Core_Session - Guard against unset value
Merge pull request #2883 from totten/4.4-lazy-session

CRM-14356 - CRM_Core_Session - Handle Drupal's lazy sessions