Do not set rec_name for unsaved record

Since 0db956eb2505 _changed_value is used for on_change_with calls, so it can
have to deal with unsaved Many2One value if it is the parent of a One2Many.
(grafted from 206e247e16437dc8b2f1989cc13b7e48a1d0dfc7)

--HG--
branch : 6.0

trytond/trytond/model/modelview.py

@@ -858,7 +858,8 @@ class ModelView(Model):
                 continue
             if field._type in ('many2one', 'one2one', 'reference'):
                 if value:
-                    if isinstance(value, ModelStorage):
+                    if (isinstance(value, ModelStorage)
+                            and value.id and value.id >= 0):
                         changed['%s.' % fname] = {
                             'rec_name': value.rec_name,
                             }