pgsql: Avoid leaking memory while reading toasted entries from

Log Message:
-----------
Avoid leaking memory while reading toasted entries from pg_rewrite,
and nail a couple more system indexes into cache. This doesn't make
any difference in normal system operation, but when forcing constant
cache resets it's difficult to get through the rules regression test
without these changes.

Modified Files:
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pgsql/src/backend/utils/cache:
relcache.c (r1.234 -> r1.235)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/relcache.c.diff?r1=1.234&r2=1.235)