CDmridDir destructor should be virtual

CDmridDir is a base class for CDmridDirHttp and CDmridDirFile, so
its destructor should be marked virtual to prevent undefined behavior.

src/cdmriddir.h

@@ -50,7 +50,7 @@ public:
     CDmridDir();
     
     // destructor
-    ~CDmridDir();
+    virtual ~CDmridDir();
     
     // init & close
     virtual bool Init(void);