Sync ChannelSendOperator copy in spring-messaging

See gh-22720

spring-messaging/src/main/java/org/springframework/messaging/handler/invocation/reactive/ChannelSendOperator.java

@@ -402,7 +402,12 @@ class ChannelSendOperator<T> extends Mono<Void> implements Scannable {
 
 		@Override
 		public void onError(Throwable ex) {
-			this.completionSubscriber.onError(ex);
+			try {
+				this.completionSubscriber.onError(ex);
+			}
+			finally {
+				this.writeBarrier.releaseCachedItem();
+			}
 		}
 
 		@Override