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/helpchm.h

@@ -63,7 +63,7 @@ protected:
     // the first 2 HtmlHelp() parameters
     bool CallHtmlHelp(unsigned cmd, WXWPARAM param)
     {
-        return CallHtmlHelp(GetParentWindow(), GetValidFilename().wx_str(),
+        return CallHtmlHelp(GetParentWindow(), GetValidFilename().t_str(),
                             cmd, param);
     }