Crash when renewing a fiscalyear without periods
When tring to renew a fiscalyear without periods the following exception is raised: ``` ZeroDivisionError: division by zero File "trytond/protocols/dispatcher.py", line 196, in _dispatch result = rpc.result(meth(*c_args, **c_kwargs)) File "trytond/wizard/wizard.py", line 340, in execute result = wizard._execute(state_name) File "trytond/wizard/wizard.py", line 375, in _execute do_result = do(action) File "trytond/modules/account/fiscalyear.py", line 693, in do_create_ fiscalyear = self.create_fiscalyear() File "trytond/modules/account_invoice/account.py", line 298, in create_fiscalyear fiscalyear = super(RenewFiscalYear, self).create_fiscalyear() File "trytond/modules/account/fiscalyear.py", line 684, in create_fiscalyear interval = months / len(periods) ```
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