contribution page with separate membership and contribution payments will create a zero $0 contribution record

On a contribution page that has a membership block, "is separate" enabled, and "Allow other amounts" enabled, if a user does not enter any additional amount, then a contribution record with the cost of the membership and and a contribution record with $0 zero cost will be added.

To recreate:
1. Create a contribution page with membership section and contribution amounts section enabled. Enable "Allow other amounts" and "Separate payments". Do not enter any values in the Fixed Contribution Options.
2. Complete the contribution form and do not enter any additional amount.
3. Review the individuals contribution record and see that there are two contribution records, one with a dollar amount of $0.

Desired behaviour:
If no additional amount is selected then only create one Contribution record in the amount of the membership.

Fix is needed to the processing logic in CRM_Member_BAO_Membership::postProcessMembership.