Add volatile keyword

Author: ngocnhan-tran1996

spring-rabbit-stream/src/main/java/org/springframework/rabbit/stream/producer/RabbitStreamTemplate.java

@@ -79,7 +79,7 @@ public class RabbitStreamTemplate implements RabbitStreamOperations, Application
 
 	private boolean streamConverterSet;
 
-	private Producer producer;
+	private volatile Producer producer;
 
 	private String beanName;