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.