pgsql: Avoid memory leak if pgstat_vacuum_stat is interrupted partway

Log Message:
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Avoid memory leak if pgstat_vacuum_stat is interrupted partway through.
The temporary hash tables made by pgstat_collect_oids should be allocated
in a short-term memory context, which is not the default behavior of
hash_create.  Noted while looking through hash_create calls in connection
with Robert Haas' recent complaint.

This is a pre-existing bug, but it doesn't seem important enough to
back-patch.  The hash table is not so large that it would matter unless this
happened many times within a session, which seems quite unlikely.

Modified Files:
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    pgsql/src/backend/postmaster:
        pgstat.c (r1.193 -> r1.194)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c?r1=1.193&r2=1.194)