vmlens: fix data race on MVStore.closed

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

@@ -149,7 +149,7 @@ public final class MVStore {
 
     private volatile boolean reuseSpace = true;
 
-    private boolean closed;
+    private volatile boolean closed;
 
     private FileStore fileStore;
     private boolean fileStoreIsProvided;