[FIX] account: prevent invoice line unlink

Nothing prevents the deletion on invoice line at the Python level. Other
modules might give access directly to the invoice lines, and make their
deletion possible when the invoice is not draft.

opw-743154

addons/account/i18n/account.pot

@@ -9467,6 +9467,12 @@ msgstr ""
 msgid "You can not delete a payment that is already posted"
 msgstr ""
 
+#. module: account
+#: code:addons/account/models/account_invoice.py:1360
+#, python-format
+msgid "You can only delete an invoice line if the invoice is in draft state."
+msgstr ""
+
 #. module: account
 #: code:addons/account/models/account_payment.py:128
 #, python-format

addons/account/models/account_invoice.py

@@ -1354,6 +1354,12 @@ class AccountInvoiceLine(models.Model):
         """
         pass
 
+    @api.multi
+    def unlink(self):
+        if self.filtered(lambda r: r.invoice_id and r.invoice_id.state != 'draft'):
+            raise UserError(_('You can only delete an invoice line if the invoice is in draft state.'))
+        return super(AccountInvoiceLine, self).unlink()
+
 class AccountInvoiceTax(models.Model):
     _name = "account.invoice.tax"
     _description = "Invoice Tax"