BUG #4715: libpq `PQgetlength' return invalid field length.

The following bug has been logged online:

Bug reference:      4715
Logged by:          VEN
Email address:      bsditer@gmail.com
PostgreSQL version: 8.3.7
Operating system:   FreeBSD
Description:        libpq `PQgetlength' return invalid field length.
Details:

Database have below table:
CREATE TABLE TEST {
    NUM BIGINT NOT NULL DEFAULT 0
};


EXEC "SELECT NUM FROM TEST LIMIT 1"
   PQgetlength return valid length (sizeof(int64)), it's ok.

but
EXEC "SELECT SUM(NUM)::BIGINT FROM TEST"
or
EXEC "SELECT SUM(NUM) FROM TEST"

   PQgetlength already return zero.