check directories passed to Append/Insert/PrependDir(); don't terminate string returned from GetPath() with wxPATH_GET_SEPARATOR flag with slash for the paths which don't have any path at all under DOS

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@24128 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/filename.h

@@ -410,6 +410,9 @@ public:
         { return GetPath(wxPATH_GET_SEPARATOR, format); }
 
 private:
+    // check whether this dir is valid for Append/Prepend/InsertDir()
+    static bool IsValidDirComponent(const wxString& dir);
+
     // the drive/volume/device specification (always empty for Unix)
     wxString        m_volume;