Defaults to current user for getting max discount

Just doing what every other dialog using CredentialsDialog does: When
validating the discount, reset self._user to api.get_current_user if it's None.
Can happen if the user runned the CredentialsDialog and then cancelled it

Change-Id: I0ff4e4b00cbc5da39b29cf1c8e53cfaa7ab155f2