Restore current hint state when returned from non-hinted query planning.

Restore current hint state when returned from non-hinted query planning.

If no hint is given for the current level query, pg_hint_plan_planner
calls the next level of planner after erasing the
current_hint_state. But it forgot to restore the state before the
planning of the rest part of the current-level query.  It is
(a-kind-of) broken by the commit d422966 but overlooked as an
inevitable side-effect of the fix.  Get back the behavior by restoring
current_hint_state after returned from the lower level planner with
unhinted query.

Issue: https://github.com/ossc-db/pg_hint_plan/issues/30
Reported-by: higuchi-daisuke