Fix potential crash when do DragNDrop in GTK

Explicit disconnect the cursor draw handler in the end of DoDragDrop to
fix potential crash cause by dangling pointer. When wxDropSource destruct,
the set of icons are gone too, so the draw handler has chances to read
dangling icons.

Author: 徐扬斌

include/wx/gtk/dnd.h

@@ -10,6 +10,7 @@
 #define _WX_GTK_DND_H_
 
 #include "wx/icon.h"
+#include "wx/gtk/private/wrapgtk.h"
 
 // ----------------------------------------------------------------------------
 // macros
@@ -107,7 +108,8 @@ class WXDLLIMPEXP_CORE wxDropSource: public wxDropSourceBase
                      m_iconNone;
 
     bool             m_waiting;
-
+    gulong           m_icon_draw_handler_id = 0;
+    GtkWidget*       m_icon_drawn_widget = nullptr;
 private:
     // common part of both ctors
     void SetIcons(const wxIcon& copy,

src/gtk/dnd.cpp

@@ -24,7 +24,6 @@
 
 #include "wx/scopeguard.h"
 
-#include "wx/gtk/private/wrapgtk.h"
 #include "wx/gtk/private/backend.h"
 
 //----------------------------------------------------------------------------
@@ -806,8 +805,8 @@ void wxDropSource::PrepareIcon( int action, GdkDragContext *context )
         cairo_region_destroy(region);
     }
 
-    g_signal_connect(widget, "draw", G_CALLBACK(draw_icon), icon);
-
+    m_icon_draw_handler_id = g_signal_connect(widget, "draw", G_CALLBACK(draw_icon), icon);
+    m_icon_drawn_widget = widget;
 #else // !__WXGTK3__
 
     GdkBitmap *mask;
@@ -919,6 +918,8 @@ wxDragResult wxDropSource::DoDragDrop(int flags)
 
     g_signal_handlers_disconnect_by_func (m_iconWindow,
                                           (gpointer) gtk_dnd_window_configure_callback, this);
+    if (m_icon_draw_handler_id > 0)
+        g_signal_handler_disconnect(m_icon_drawn_widget, m_icon_draw_handler_id);
     g_object_unref(m_iconWindow);
     m_iconWindow = NULL;