use-after-free in jsonb_set_lax()

I received an off-list report of a crash in production using this
function. It's possible to reproduce a crash in a non-assert build
with a fuzzing script, but the easiest demo of the problem is (on an
assert build):

=# SELECT jsonb_set_lax(j, '{a}', NULL, true, t)
FROM (VALUES (jsonb_build_object('a', '1'), 'use_json_null'),
(jsonb_build_object('a', '1'), 'delete_key')) AS v(j, t);
ERROR: unknown type of jsonb container

I've attached a fix, which I plan to commit to all supported versions
in a couple days barring objections.

I didn't find any other places in the code base with a similar bug,
but of course I can't be certain.

--
John Naylor
Amazon Web Services