Fix old bug in log_autovacuum_min_duration code: it was relying on being able

Fix old bug in log_autovacuum_min_duration code: it was relying on being able
to access a Relation entry it had just closed.  I happened to be testing with
CLOBBER_CACHE_ALWAYS, which made this a guaranteed core dump (at least on
machines where sprintf %s isn't forgiving of a NULL pointer).  It's probably
quite unlikely that it would fail in the field, but a bug is a bug.  Fix by
moving the relation_close call down past the logging action.