[sqlsmith] Crash on GUC serialization

Hi,

sqlsmith just made a GUC that tricks the serialization code into
dereferencing a nullpointer.  Here's a recipe:

--8<---------------cut here---------------start------------->8---
set min_parallel_relation_size to 0;
set max_parallel_workers_per_gather to 2;
set force_parallel_mode to on;
begin;
select set_config('foo.baz', null, b) from (values (false), (true)) g(b);
commit;
select 1;
--8<---------------cut here---------------end--------------->8---

regards,
Andreas