Log Message:
-----------
Avoid crash in interrupted autovacuum worker, caused by leaving the current
memory context pointing at a context not long lived enough.
Also, create a fake PortalContext where to store the vac_context, if only
to avoid having it be a top-level memory context.
Modified Files:
--------------
pgsql/src/backend/postmaster:
autovacuum.c (r1.52 -> r1.53)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c.diff?r1=1.52&r2=1.53)