CRM-13995 - Add is_object check to prevent notices.

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* CRM-13995: contact export when merging households: some household fields missing
  http://issues.civicrm.org/jira/browse/CRM-13995
Merge pull request #2301 from davecivicrm/CRM-13995a

CRM-13995 - Add is_object check to prevent notices.