Problem with monetary value format in membership renewal

When renewing a membership I got the error "Please enter a valid amount." even though I didn't check the option "Record Renewal Payment".
The reason was that the amount is filled in by default without applying the configured monetary value format. After form submission the amount is checked for the correct format even if no payment is recorded.

The attached patch applies the monetary value format to the amount. The amount is still checked even if no payment is being recorded, but does't result in an error anymore.
The patch was made against CiviCRM v3.3.3