Added check that an order exists in OrderConfirmView

The `OrderConfirmView` assumes that there is an order set on the request.  This isn't necessarily the case, and if there isn't it triggers a server error:

    'NoneType' object has no attribute 'status'

This is a slightly difficult bug to spot, as if it works the first time for a user/developer then it sets a permanent redirect to the next stage of the payment.  I was able to reproduce it by visiting the correct url using a private browsing window in Firefox.