Prevent to enter invoice line if company is not set

The company of invoice line is set by the domain from invoice lines. So if the
field is empty, the invoice line is not valid because company is required.

issue6762
review38681002

invoice.py

@@ -154,7 +154,10 @@ class Invoice(Workflow, ModelSQL, ModelView, TaxableMixin):
         domain=[
             ('company', '=', Eval('company', -1)),
             ],
-        states=_STATES, depends=['state', 'currency_date', 'company'])
+        states={
+            'readonly': (Eval('state') != 'draft') | ~Eval('company'),
+            },
+        depends=['state', 'company'])
     taxes = fields.One2Many('account.invoice.tax', 'invoice', 'Tax Lines',
         states=_STATES, depends=_DEPENDS)
     comment = fields.Text('Comment', states=_STATES, depends=_DEPENDS)