Set `instance` to `null` after closing it (#163)

diff --git a/commons-vfs2/src/main/java/org/apache/commons/vfs2/VFS.java b/commons-vfs2/src/main/java/org/apache/commons/vfs2/VFS.java
index 43250aa..9769bb3 100644
--- a/commons-vfs2/src/main/java/org/apache/commons/vfs2/VFS.java
+++ b/commons-vfs2/src/main/java/org/apache/commons/vfs2/VFS.java
@@ -40,6 +40,7 @@
     public static synchronized void close() {
         if (instance != null) {
             instance.close();
+            instance = null;
         }
     }