message: correct an issue of trackbar on macOS
  Python binding is passing NULL as (int*)value.
  sliderChanged crash when trying to dereference value
diff: @@ -1201,7 +1201,7 @@ static NSSize constrainAspectRatio(NSSize base, NSSize constraint) {
  (void)notification;
  int pos = [slider intValue];
  NSString *temp = [self initialName];
  - NSString *text = [NSString stringWithFormat:@"%@ %d", temp, *value];
  + NSString *text = [NSString stringWithFormat:@"%@ %d", temp, pos];
  [name setStringValue: text];
  if(value)
  *value = pos;
  