fix found by TestMVTableEngine in multithreaded mode

h2/src/main/org/h2/mvstore/MVMap.java

@@ -60,7 +60,9 @@ public class MVMap<K, V> extends AbstractMap<K, V>
     private ConcurrentArrayList<Page> oldRoots =
             new ConcurrentArrayList<Page>();
 
-    private boolean closed;
+    
+    /** volatile so we don't accidentally write to a closed map in multithreaded mode */
+    private volatile boolean closed;
     private boolean readOnly;
     private boolean isVolatile;