Return an empty string from wxDataFormat::GetId if there was an error

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@44278 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/ole/dataobj.cpp

@@ -159,6 +159,7 @@ wxString wxDataFormat::GetId() const
     if ( !len )
     {
         wxLogError(_("The clipboard format '%d' doesn't exist."), m_format);
+        return wxEmptyString;
     }
 
     return s;