set richedit DLL handle to 0 after unloading (patch 768994)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@22061 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/textctrl.cpp

@@ -2234,6 +2234,7 @@ void wxRichEditModule::OnExit()
         if ( ms_hRichEdit[i] )
         {
             ::FreeLibrary(ms_hRichEdit[i]);
+            ms_hRichEdit[i] = NULL;
         }
     }
 }