Avoid PQisBusy/PQconsumeInput busy loop in case of PQisBusy returning

Avoid PQisBusy/PQconsumeInput busy loop in case of PQisBusy returning
false. per Tom Lane's suggestion. See:

Subject: Suggested change to pgbench
From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tatsuo Ishii <t-ishii@sra.co.jp>
Cc: pgsql-patches@postgreSQL.org
Date: Sun, 06 Oct 2002 12:37:27 -0400

for more details.