pgsql: Fix a small memory leak in ExecReScanAgg() in the hashed

Log Message:
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Fix a small memory leak in ExecReScanAgg() in the hashed aggregation case.
In the previous coding, the list of columns that needed to be hashed on
was allocated in the per-query context, but we reallocated every time
the Agg node was rescanned. Since this information doesn't change over
a rescan, just construct the list of columns once during ExecInitAgg().

Modified Files:
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    pgsql/src/backend/executor:
        nodeAgg.c (r1.161 -> r1.162)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeAgg.c?r1=1.161&r2=1.162)