Add missing parenthesis on test in O2M.set_on_change

issue2917

tryton/gui/window/view_form/model/field.py

@@ -707,7 +707,7 @@ class O2MField(CharField):
             record.value[self.name].remove(record2, signal=False,
                 force_remove=True)
 
-        if value and value.get('add') or value.get('update', []):
+        if value and (value.get('add') or value.get('update', [])):
             record.value[self.name].add_fields(fields, signal=False)
             for vals in value.get('add', []):
                 new_record = record.value[self.name].new(default=False)