pgsql: Fix finalization for json_objectagg and friends

Fix finalization for json_objectagg and friends

Commit f4fb45d15c misguidedly tried to free some state during aggregate
finalization for json_objectagg. This resulted in attempts to access
freed memory, especially when the function is used as a window function.
Commit 4eb9798879 attempted to ameliorate that, but in fact it should
just be ripped out, which is done here. Also add some regression tests
for json_objectagg in various flavors as a window function.

Original report from Jaime Casanova, diagnosis by Andres Freund.

Discussion: /messages/by-id/YkfeMNYRCGhySKyg@ahch-to

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/112fdb3528465cc14a2f1dff3dc27f100326d885

Modified Files
--------------
src/backend/utils/adt/json.c | 12 ------------
src/test/regress/expected/sqljson.out | 35 +++++++++++++++++++++++++++++++++++
src/test/regress/sql/sqljson.sql | 18 ++++++++++++++++++
3 files changed, 53 insertions(+), 12 deletions(-)