pgsql: Avoid double transformation of json_array()'s subquery.

Avoid double transformation of json_array()'s subquery.

transformJsonArrayQueryConstructor() applied transformStmt() to
the same subquery tree twice. While this causes no issue in many
cases, there are some where it causes a coredump, thanks to the
parser's habit of scribbling on its input.

Fix by making a copy before the first transformation (compare
0f43083d1). This is quite brute-force, but then so is the
whole business of transforming the input twice. Per discussion
in the bug thread, this implementation of json_array() parsing
should be replaced completely. But that will take some work
and will surely not be back-patchable, so for the moment let's
take the easy way out.

Oversight in 7081ac46a. Back-patch to v16 where that came in.

Bug: #18877
Reported-by: Yu Liang <luy70(at)psu(dot)edu>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/18877-c3c3ad75845833bb@postgresql.org
Backpatch-through: 16

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ca54f9b706f66f8f7191551aec8eac8b86b0fb9b

Modified Files
--------------
src/backend/parser/parse_expr.c | 2 +-
src/test/regress/expected/sqljson.out | 6 ++++++
src/test/regress/sql/sqljson.sql | 2 ++
3 files changed, 9 insertions(+), 1 deletion(-)