[6.0] product_id_change of account.invoice may fail if no company_id or currency_id is supplied.

On trunk/6.0 the account module was updated to add multicompany support. The invoices product_id field 'on-change' method (named product_id_change) was modified to get the company_id from the context and the invoice view was modified accordingly to pass the company_id on the context.
But if a module does not pass the company_id on the context (like account_tax_include, see bug 615324) then product_id_change raises an exception cause it does not verify properly whether company_id is defined.

Currently the code looks like this:

    def product_id_change(self, cr, uid, ids, product, uom, qty=0, name='', type='out_invoice', partner_id=False, fposition_id=False, price_unit=False, address_invoice_id=False, currency_id=False, context=None):
        ...
        company_id = context.get('company_id',False)
        ...
        if company_id:
            ...
        ...
        if not company_id and not currency_id:
            return res_final

        company = self.pool.get('res.company').browse(cr, uid, company_id)
        currency = self.pool.get('res.currency').browse(cr, uid, currency_id)
        ...

That means that, if currency_id is defined, but company_id is not defined the method will crash!
Also, if no currency_id is defined, but company_id is in context, it will crash too.

Instead of "if not company_id and not currency_id:" it should be "if not company_id or not currency_id:".