CRM-13432 - Fix notices for Fixed Period memberships.

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* CRM-13432: Creating membershiptype with period fixed and start date/rolling date gives warning.
  http://issues.civicrm.org/jira/browse/CRM-13432
Merge pull request #1677 from davecivicrm/CRM-13432

CRM-13432 - Fix notices for Fixed Period memberships.