Ignore missing STOMP decoder if session closed

Closes gh-24842

Author: rstoyanchev

spring-websocket/src/main/java/org/springframework/web/socket/messaging/StompSubProtocolHandler.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -240,6 +240,10 @@ else if (webSocketMessage instanceof BinaryMessage) {
 
 			BufferingStompDecoder decoder = this.decoders.get(session.getId());
 			if (decoder == null) {
+				if (!session.isOpen()) {
+					logger.trace("Dropped inbound WebSocket message due to closed session");
+					return;
+				}
 				throw new IllegalStateException("No decoder for session id '" + session.getId() + "'");
 			}