Check CREATE_REPLICATION_SLOT response shape in libpqwalreceiver

Check CREATE_REPLICATION_SLOT response shape in libpqwalreceiver

Previously, libpqrcv_create_slot() checked only that
CREATE_REPLICATION_SLOT returned PGRES_TUPLES_OK before reading
values from the first row. If the server unexpectedly returned an
invalid result, such as zero rows, PQgetvalue() could return NULL,
leading to a crash while parsing the LSN.

Other replication commands, such as IDENTIFY_SYSTEM, already validate
the response shape before accessing result values, but
CREATE_REPLICATION_SLOT did not.

Fix this by verifying that CREATE_REPLICATION_SLOT response contains
exactly one row with four fields, and report a protocol violation otherwise.

Backpatch to all supported versions.

Bug: #19547
Reported-by: Yuelin Wang <1217816127@qq.com>
Author: Kenny Chen <kennychen851228@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/19547-f7986f668f71e788@postgresql.org
Discussion: https://postgr.es/m/CAPXstDtW2iqe+DJAOTQTX+rRziJp2UhZSo1+HRj1COAtbu+nKw@mail.gmail.com
Backpatch-through: 14