pgsql: Fix one more place where we were expecting lcons() to be

Log Message:
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Fix one more place where we were expecting lcons() to be nondestructive
to the original List; per report from Sebastian B�ck.  I think this is
the last such bug --- I examined every lcons() call in the backend and
the rest seem OK --- but it's nervous-making that we're still finding
'em so many months after the List rewrite went in.

Modified Files:
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    pgsql/src/backend/rewrite:
        rewriteHandler.c (r1.145 -> r1.146)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/rewrite/rewriteHandler.c.diff?r1=1.145&r2=1.146)