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

src/msw/combobox.cpp

@@ -308,7 +308,7 @@ bool wxComboBox::MSWCommand(WXUINT param, WXWORD id)
             // this string is going to become the new combobox value soon but
             // we need it to be done right now, otherwise the event handler
             // could get a wrong value when it calls our GetValue()
-            ::SetWindowText(GetHwnd(), value.wx_str());
+            ::SetWindowText(GetHwnd(), value.t_str());
 
             {
                 wxCommandEvent event(wxEVT_COMMAND_COMBOBOX_SELECTED, GetId());