plpsm - plpgpsm: All composite types are nullable in SQL/PSM and default

Log Message:
-----------
All composite types are nullable in SQL/PSM and default value is NULL. PostgreSQL
doesn't support objects constructor, so value has to be inicialised with row constructor.

CREATE TYPE xx AS (a integer, b integer);
CREATE OR REPLACE FUNCTION fxx()
RETURNS void AS
$$
BEGIN
DECLARE a xx;
IF a IS NULL THEN PRINT 'is null'; END IF;
SET a = row();
SET a.a = 10, a.b = 20;
PRINT a;
SET a = NULL;
PRINT a;
END;
$$ LANGUAGE plpgpsm;
SELECT fxx();
NOTICE: is null
NOTICE: (10,20)
NOTICE: <NULL>
fxx
-----

(1 row)

DROP FUNCTION fxx();
-- cannot use empty composite value
CREATE OR REPLACE FUNCTION fxx()
RETURNS void AS
$$
BEGIN
DECLARE a xx;
PRINT a.a;
END;
$$ LANGUAGE plpgpsm;
SELECT fxx();
ERROR: variable "a" is not assigned yet
CONTEXT: PL/pgPSM function "fxx()" line 4 at PRINT
DROP FUNCTION fxx();
DROP TYPE xx;

Modified Files:
--------------
plpgpsm/src:
pl_comp.c (r1.26 -> r1.27)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/plpsm/plpgpsm/src/pl_comp.c.diff?r1=1.26&r2=1.27)
pl_exec.c (r1.62 -> r1.63)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/plpsm/plpgpsm/src/pl_exec.c.diff?r1=1.62&r2=1.63)
plpgpsm.h (r1.48 -> r1.49)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/plpsm/plpgpsm/src/plpgpsm.h.diff?r1=1.48&r2=1.49)
plpgpsm/src/expected:
plpgpsm.out (r1.22 -> r1.23)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/plpsm/plpgpsm/src/expected/plpgpsm.out.diff?r1=1.22&r2=1.23)
plpgpsm/src/sql:
plpgpsm.sql (r1.19 -> r1.20)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/plpsm/plpgpsm/src/sql/plpgpsm.sql.diff?r1=1.19&r2=1.20)