Fix crash when destroying wxPopupWindow without hiding it first

The global wxCurrentPopupWindow used by wxMSW wasn't reset in this case
and resulted in a crash during the next mouse or focus event.

Do it in wxPopupWindow dtor now, and not just when the popup is hidden.

include/wx/msw/popupwin.h

@@ -25,6 +25,8 @@ public:
 
     bool Create(wxWindow *parent, int flags = wxBORDER_NONE);
 
+    virtual ~wxPopupWindow();
+
     virtual void SetFocus() wxOVERRIDE;
     virtual bool Show(bool show = true) wxOVERRIDE;