Fix encoding conversion function bug.

See following posting for more details.

Subject: Re: [HACKERS] [BUGS] Bug #943: Server-Encoding from EUC_TW to UTF-8 doesn't
From: Tatsuo Ishii <t-ishii@sra.co.jp>
To: michael.enke@wincor-nixdorf.com, pgsql-bugs@postgresql.org
Cc: pgsql-hackers@postgresql.org
Date: Sat, 12 Apr 2003 10:51:45 +0900 (JST)