pgsql: Fix window function plan generation to cope with volatile sort

Log Message:
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Fix window function plan generation to cope with volatile sort expressions.
(Not clear how useful these really are, but failing is no good...)
Per report from David Fetter and Robert Treat.

Modified Files:
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    pgsql/src/backend/optimizer/plan:
        planner.c (r1.252 -> r1.253)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/planner.c?r1=1.252&r2=1.253)