pgsql: Replace the use of "0" with "NULL" where appropriate in dllist.c,

Log Message:
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Replace the use of "0" with "NULL" where appropriate in dllist.c, for
good style and to satisfy sparse. From Alvaro Herrera.

Modified Files:
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pgsql/src/backend/lib:
dllist.c (r1.30 -> r1.31)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/lib/dllist.c.diff?r1=1.30&r2=1.31)