CRM-13108 - Fix warning in CRM_Utils_File::findFiles (from rubofvil)

Note: According to http://php.net/glob, the return result varies by
platform; sometimes it returns array(), and sometimes it returns FALSE.
The patch should work with either case.

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* CRM-13108: Warnings when extensions is installed
  http://issues.civicrm.org/jira/browse/CRM-13108