[FIX] marketing_campaign: error during migration

Without this patch, there's a traceback when creating a segment due
to an onchange trying to set an attribute `ir_filter_id` to a dict.

related to rev 0fa2ac1d5d

addons/marketing_campaign/models/marketing_campaign.py

@@ -200,7 +200,7 @@ class MarketingCampaignSegment(models.Model):
         if model:
             res['domain']['ir_filter_id'] = [('model_id', '=', model)]
         else:
-            res.ir_filter_id = False
+            self.ir_filter_id = False
         return res
 
     @api.multi