add channel is closed check

Original commit: elastic/x-pack-elasticsearch@04bcdf8308

elasticsearch/x-pack/security/src/main/java/org/elasticsearch/xpack/security/transport/ServerTransportFilter.java

@@ -91,10 +91,12 @@ public interface ServerTransportFilter {
                 } else if (((TcpTransportChannel) unwrappedChannel).getChannel() instanceof io.netty.channel.Channel) {
                     io.netty.channel.Channel channel = (io.netty.channel.Channel) ((TcpTransportChannel) unwrappedChannel).getChannel();
                     io.netty.handler.ssl.SslHandler sslHandler = channel.pipeline().get(io.netty.handler.ssl.SslHandler.class);
+                    if (channel.isOpen()) {
                         assert sslHandler != null : "channel [" + channel + "] did not have a ssl handler. pipeline " + channel.pipeline();
                         extactClientCertificates(sslHandler.engine(), channel);
                     }
                 }
+            }
 
             Authentication authentication = authcService.authenticate(securityAction, request, null);
             authzService.authorize(authentication, securityAction, request);