pgsql: ActiveSnapshot must be set to something valid while running

Log Message:
-----------
ActiveSnapshot must be set to something valid while running deferred
triggers during COMMIT. Per trouble report from Frank van Vugt.

Modified Files:
--------------
pgsql/src/backend/commands:
trigger.c (r1.175 -> r1.176)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/trigger.c.diff?r1=1.175&r2=1.176)