fix crash caused by an wxMDIClientWindow's GTK signal handler being called when the wxMDIClientWindow is already (partially) destroyed: easily reproducible (without this fix) in the 'mdi' and 'svg' samples

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@59523 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/mdi.h

@@ -140,6 +140,7 @@ class WXDLLIMPEXP_CORE wxMDIClientWindow : public wxMDIClientWindowBase
 {
 public:
     wxMDIClientWindow() { }
+    ~wxMDIClientWindow();
 
     virtual bool CreateClient(wxMDIParentFrame *parent,
                               long style = wxVSCROLL | wxHSCROLL);