Fix incorrect error message in libpq_pipeline

Michael Paquier <michael@paquier.xyz>

Commit: 4fd633df5080ef45b1a1665e6599463aad42305a
Author: Michael Paquier <michael@paquier.xyz>
Date: 2023-06-23T08:50:23Z
Releases: 15.4
Fix incorrect error message in libpq_pipeline

One of the tests for the pipeline mode with portal description expects a
non-NULL PQgetResult, but used an incorrect error message on failure,
telling that PQgetResult being NULL was the expected result.

Author: Jelte Fennema
Discussion: https://postgr.es/m/CAGECzQTkShHecFF+EZrm94Lbsu2ej569T=bz+PjMbw9Aiioxuw@mail.gmail.com
Backpatch-through: 14

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