Fix a rare race condition on shutdown:

If we happen to get very unlucky and close the door when no
accept operation is pending, the do_accept loop would never
terminate.

src/ripple/server/impl/Door.h

@@ -369,7 +369,7 @@ void
 Door<Handler>::
 do_accept(boost::asio::yield_context do_yield)
 {
-    for(;;)
+    while (acceptor_.is_open())
     {
         error_code ec;
         endpoint_type remote_address;