Fix a tiny memory leak (one List header) in RelationCacheInvalidate().

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?