fix double checked locking with volatile (#649)

spring-cloud-context/src/main/java/org/springframework/cloud/context/scope/GenericScope.java

@@ -365,7 +365,7 @@ public class GenericScope implements Scope, BeanFactoryPostProcessor,
 
 		private final ObjectFactory<?> objectFactory;
 
-		private Object bean;
+		private volatile Object bean;
 
 		private Runnable callback;