pgsql-server: Add some marginal tweaks to eliminate memory leakages

Log Message:
-----------
Add some marginal tweaks to eliminate memory leakages associated with
subtransactions. Trivial subxacts (such as a plpgsql exception block
containing no database access) now demonstrably leak zero bytes.

Modified Files:
--------------
pgsql-server/src/backend/access/transam:
xact.c (r1.189 -> r1.190)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/xact.c.diff?r1=1.189&r2=1.190)
pgsql-server/src/backend/executor:
spi.c (r1.128 -> r1.129)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/executor/spi.c.diff?r1=1.128&r2=1.129)
pgsql-server/src/backend/utils/mmgr:
aset.c (r1.57 -> r1.58)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/utils/mmgr/aset.c.diff?r1=1.57&r2=1.58)
mcxt.c (r1.50 -> r1.51)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/utils/mmgr/mcxt.c.diff?r1=1.50&r2=1.51)
pgsql-server/src/include/nodes:
memnodes.h (r1.28 -> r1.29)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/nodes/memnodes.h.diff?r1=1.28&r2=1.29)
pgsql-server/src/include/utils:
memutils.h (r1.57 -> r1.58)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/utils/memutils.h.diff?r1=1.57&r2=1.58)