CRM-12849, CRM-12274 prevent log reports from fatal

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* CRM-12849: "Change log" tab crashes with "Table log_civicrm_activity_target doesn't exist"
  http://issues.civicrm.org/jira/browse/CRM-12849
* CRM-12274: Merge activity tables into one unified table activity contact table
  http://issues.civicrm.org/jira/browse/CRM-12274