The total amount is calculated wrong when separate membership is enabled

When contribution page has separate membership enabled then the total amount changes after confirmation.

To replicate
1. Enable separate membership payment for a contribution page with other amount enabled.
2. Sign-up for a membership http://drupal.sandbox.civicrm.org/civicrm/contribute/transact?reset=1&id=2 and select 'Student' membership type and add some additional amount.

The confirmation page will populate correct amount but the thank you page will change the amount for membership type and the contribution is recorded with changed amount. It always adds minimum fee of 1st membership type in a system irrespective of selection of membership type.

I did some debugging, the commit https://github.com/civicrm/civicrm-core/commit/5624f515 has caused the regression. The code at line 1706 in CRM/Contribute/Form/Contribution/Confirm.php is buggy.