pgsql: Fix a tiny memory leak (one List header) in

Log Message:
-----------
Fix a tiny memory leak (one List header) in RelationCacheInvalidate().
This is utterly insignificant in normal operation, but it becomes a
problem during cache inval stress testing.  The original coding in fact
had no leak --- the 8.0 List rewrite created the issue.  I wonder whether
list_concat should pfree the discarded header?

Modified Files:
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    pgsql/src/backend/utils/cache:
        relcache.c (r1.235 -> r1.236)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/relcache.c.diff?r1=1.235&r2=1.236)