when closing an MVStore we should clear the filechannel to prevent

(a) any accidental further access to it
(b) throwing an exception if the client code calls close() more than once

h2/src/test/org/h2/test/store/TestMVStore.java

@@ -50,6 +50,7 @@ public class TestMVStore extends TestBase {
         testRandom();
         testKeyValueClasses();
         testIterate();
+        testCloseTwice();
         testSimple();
     }
 
@@ -771,6 +772,20 @@ public class TestMVStore extends TestBase {
         s.close();
     }
 
+    private void testCloseTwice() {
+        String fileName = getBaseDir() + "/testCloseTwice.h3";
+        FileUtils.delete(fileName);
+        MVStore s = openStore(fileName);
+        MVMap<Integer, String> m = s.openMap("data", Integer.class, String.class);
+        for (int i = 0; i < 3; i++) {
+            m.put(i, "hello " + i);
+        }
+        s.store();
+        // closing twice should be fine
+        s.close();
+        s.close();
+    }
+    
     private void testSimple() {
         String fileName = getBaseDir() + "/testSimple.h3";
         FileUtils.delete(fileName);

h2/src/tools/org/h2/dev/store/btree/MVStore.java

@@ -450,8 +450,9 @@ public class MVStore {
                 maps.clear();
                 mapsChanged.clear();
             } catch (Exception e) {
-                file = null;
                 throw convert(e);
+            } finally {
+                file = null;
             }
         }
     }