CRM-13957 - import - undefined index when mapping fields

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* CRM-13957: 2nd page of Import script reports undefined index
  http://issues.civicrm.org/jira/browse/CRM-13957
CRM-13957 - import - avoid non-object error

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* CRM-13957: 2nd page of Import script reports undefined index
  http://issues.civicrm.org/jira/browse/CRM-13957
Merge pull request #2198 from jmcclelland/undefined-index-import

CRM-13957 - import - undefined index when mapping fields