Crash when scanning BR0200000000010670000117668C1

[nulliksvierde]

Hi!

We faced a crash when trying to scan the specific (brasilian) IBAN "BR0200000000010670000117668C1":

Fatal error: Unexpectedly found nil while unwrapping an Optional value: file RFIBANHelper/ISO7064.swift, line 31

We found the IBAN in an example list here: https://www.iban.de/iban-laenderliste.html

Simply checking the result of Int(remainingInput) for a nil value before force unwrapping it seems to be a valid quickfix to prevent the crash, but since this still won't allow us to scan the IBAN I'm sure there must be a nicer solution ;)