make closed flag volatile as suggested by Dawid Weiss - COMPRESS-297

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/compress/trunk@1650873 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/src/main/java/org/apache/commons/compress/archivers/zip/ZipFile.java b/src/main/java/org/apache/commons/compress/archivers/zip/ZipFile.java
index 13dff05..348d86b 100644
--- a/src/main/java/org/apache/commons/compress/archivers/zip/ZipFile.java
+++ b/src/main/java/org/apache/commons/compress/archivers/zip/ZipFile.java
@@ -134,7 +134,7 @@
     /**
      * Whether the file is closed.
      */
-    private boolean closed = true;
+    private volatile boolean closed = true;
 
     // cached buffers - must only be used locally in the class (COMPRESS-172 - reduce garbage collection)
     private final byte[] DWORD_BUF = new byte[DWORD];