[lazarus] listbox patch

[Micha Nelissen]

Vincent Snijders wrote:

> Attached patch enables storing objects in a listbox or checklistbox. The
> problems I predicted with the implementation came true. In some
> applications the destructor of the list called first and after that the
> LCL destroys the handle, sometimes it is the other way around. I think I
> have captured both cases in the patch.

Good work there, the patch looks good. I added a check for nil result.

Thanks,

Micha.