bug fix for issue 1739, two changes included:

* fix typo in OC_Group_Database::DisplayNamesInGroup's SQL clause
* check array_diff return value in OC_Group::displayNamesInGroups,
  when there is no difference between two arrays, it will return
  NULL, so we have to take care of it.
Merge pull request #1768 from houqp/issue_1739

bug fix for issue 1739