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

src/test/modules/libpq_pipeline/libpq_pipeline.c

@@ -959,7 +959,7 @@ test_prepared(PGconn *conn)
 		pg_fatal("pipeline sync failed: %s", PQerrorMessage(conn));
 	res = PQgetResult(conn);
 	if (res == NULL)
-		pg_fatal("expected NULL result");
+		pg_fatal("PQgetResult returned null");
 	if (PQresultStatus(res) != PGRES_COMMAND_OK)
 		pg_fatal("expected COMMAND_OK, got %s", PQresStatus(PQresultStatus(res)));