When saving record don't store target link for one2many to create

When saving an instance if the relation field of a one2many item is set, it
should not try to save it as it will be replaced by the current record. And in
the worst case, the target set could not be valid and so it prevents to store
the instance which doesn't need of this target.

issue3501
review3601003

trytond/model/modelstorage.py

@@ -1382,6 +1382,9 @@ class ModelStorage(Model):
                 to_write = []
                 for target in targets:
                     if target.id < 0:
+                        if field._type == 'one2many':
+                            # Don't store old target link
+                            setattr(target, field.field, None)
                         to_create.append(target._save_values)
                     else:
                         if target.id in to_remove: