4465490: Suspicious about double-check locking idiom being used in the code

Summary: to use volatile for the double-checked object
Reviewed-by: weijun

src/share/classes/java/util/jar/JarVerifier.java

@@ -76,7 +76,7 @@ class JarVerifier {
     private ByteArrayOutputStream baos;
 
     /** The ManifestDigester object */
-    private ManifestDigester manDig;
+    private volatile ManifestDigester manDig;
 
     /** the bytes for the manDig object */
     byte manifestRawBytes[] = null;