Use utf8_str(), not mb_str(), for strings passed to GTK+.

All GTK+ strings must be encoded in UTF-8, not whichever encoding the current
locale happens to use.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72545 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/utilsgtk.cpp

@@ -319,11 +319,13 @@ protected:
 
         // append this stack frame's info in the dialog
         if (!frame.GetFileName().empty() || !fncname.empty())
+        {
             gtk_assert_dialog_append_stack_frame(m_dlg,
-                                                fncname.mb_str(),
+                                                fncname.utf8_str(),
-                                                frame.GetFileName().mb_str(),
+                                                frame.GetFileName().utf8_str(),
                                                 frame.GetLine());
         }
+    }
 
 private:
     GtkAssertDialog *m_dlg;