pgsql: Avoid crash in interrupted autovacuum worker, caused by leaving

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)