added volatile to shared sazon processor

Author: Paurikova2

dspace-oai/src/main/java/org/dspace/xoai/services/impl/resources/SharedSaxonProcessor.java

@@ -24,8 +24,8 @@
  */
 public class SharedSaxonProcessor {
 
-    private static SaxonTransformerFactory saxonTransformerFactory;
-    private static Processor sharedProcessor;
+    private static volatile SaxonTransformerFactory saxonTransformerFactory;
+    private static volatile Processor sharedProcessor;
 
     /**
      * Initialize the shared processor with the given TransformerFactory.