Fix wxBase compilation with UTF-8-based wxString under MSW.

Use wxString::t_str() instead of wx_str() in Windows API function calls.

Closes #14325.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71546 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/appbase.cpp

@@ -1190,7 +1190,7 @@ bool DoShowAssertDialog(const wxString& msg)
               wxT("You can also choose [Cancel] to suppress ")
               wxT("further warnings.");
 
-    switch ( ::MessageBox(NULL, msgDlg.wx_str(), wxT("wxWidgets Debug Alert"),
+    switch ( ::MessageBox(NULL, msgDlg.t_str(), wxT("wxWidgets Debug Alert"),
                           MB_YESNOCANCEL | MB_ICONSTOP ) )
     {
         case IDYES: