[base_iban] Error on IBAN when configuring a bank account

[Christophe Combelles]

This is on 05/10/2012 nightly of the 6.1 branch.

How to reproduce :

- install a fresh database with fr_FR language
- switch the user to the "extended" interface
- Install account_voucher
- Choose the French accounting
- go to configuration → Company → Your company → Bank accounts → Add an account...
- Choose IBAN (1st in the list), choose Your Company as owner, leave the acc_number empty, fill in the bank_bic
- Save and close the wizard
- Save the Company... :

Traceback (most recent call last):
(...)
File "/home/dadafkas/projets/anybox/buildout/parts/openerp-6.1-20121005-233110/openerp/addons/base_iban/base_iban.py", line 115, in is_iban_valid
    return int(iban2) % 97 == 1
ValueError: invalid literal for int() with base 10: ''