Fix for CRM-14354 - Additional gift breaks ARB (auto-renewing) memberships.

This pull request addresses the bug in the "separate membership payment" context. When "separate membership payment" is not checked, the bug persists. Presently, the result is that combined payments don't renew/recur at all.

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* CRM-14354: Additional gift breaks ARB (auto-renewing) memberships
  https://issues.civicrm.org/jira/browse/CRM-14354