The autofill::CreditCard class allows 0 as a valid month value, which basically acts as a no-data value. Since this is also allowed as a valid expiration date by autofill::IsValidCreditCardExpirationDate, it's possible to import a credit card from a form where the month value is "" or "0".

IsValidCreditCardExpirationDate should consider 0 to be an invalid date so that we don't offer to save the card in this case.