[IMP] mrp: done unbuild order locked

This commit doesn't allow to cancel an unbuild order if the state is
'done'

PR:22085
TASK:38933

addons/mrp/models/mrp_unbuild.py

@@ -82,6 +82,13 @@ class MrpUnbuild(models.Model):
             vals['name'] = self.env['ir.sequence'].next_by_code('mrp.unbuild') or _('New')
         return super(MrpUnbuild, self).create(vals)
 
+    @api.multi
+    def unlink(self):
+        if 'done' in self.mapped('state'):
+            raise UserError(_('You cannot delete an unbuild order if the state'
+            'is "Done"'))
+        return super(MrpUnbuild, self).unlink()
+
     @api.multi
     def action_unbuild(self):
         self.ensure_one()