pgsql: Fix incorrect error message for IDENTIFY_SYSTEM

Fix incorrect error message for IDENTIFY_SYSTEM

Commit 5a991ef8692e accidentally reversed the order of the tuples
and fields parameters, making the error message incorrectly refer
to 3 tuples with 1 field when IDENTIFY_SYSTEM returns 1 tuple and
3 or 4 fields. Fix by changing the order of the parameters. This
also adds a comment describing why we check for < 3 when postgres
since 9.4 has been sending 4 fields.

Backpatch all the way since the bug is almost a decade old.

Author: Tomonari Katsumata <t(dot)katsumata1122(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Bug: #18224
Backpatch-through: v12

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b608afe980a5dfe91d76148900179d52d4db5e64

Modified Files
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src/backend/replication/libpqwalreceiver/libpqwalreceiver.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)