pgsql: PersistHoldablePortal must establish the correct value for

Log Message:
-----------
PersistHoldablePortal must establish the correct value for ActiveSnapshot
while completing execution of the cursor's query.  Otherwise we get wrong
answers or even crashes from non-volatile functions called by the query.
Per report from andrew@supernews.

Modified Files:
--------------
    pgsql/src/backend/commands:
        portalcmds.c (r1.39 -> r1.40)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/portalcmds.c.diff?r1=1.39&r2=1.40)