pgsql: Fix inappropriately-timed memory context switch in

Log Message:
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Fix inappropriately-timed memory context switch in autovacuum_do_vac_analyze.
This accidentally failed to fail before 8.3, because the context we were
switching back to was long-lived anyway; but it sure looks risky as can be
now. Well spotted by Pavan Deolasee.

Modified Files:
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pgsql/src/backend/postmaster:
autovacuum.c (r1.73 -> r1.74)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c?r1=1.73&r2=1.74)