Use wxString::t_str() in calls to Windows API functions in wxMSW.

Use t_str() instead of wx_str() to make the code work correctly in UTF-8 build
in which wx_str() returns a pointer to UTF-8 buffer while we need a wchar_t
pointer for Windows.

Closes #14371.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71640 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/private.h

@@ -796,7 +796,7 @@ public:
     {
         if ( IsRegistered() )
         {
-            if ( !::UnregisterClass(m_clsname.wx_str(), wxGetInstance()) )
+            if ( !::UnregisterClass(m_clsname.t_str(), wxGetInstance()) )
             {
                 wxLogLastError(wxT("UnregisterClass"));
             }