don't leak client data (1st part of patch 646145)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@18067 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/univ/listbox.cpp

@@ -127,6 +127,8 @@ bool wxListBox::Create(wxWindow *parent,
 
 wxListBox::~wxListBox()
 {
+    // call this just to free the client data -- and avoid leaking memory
+    DoClear();
 }
 
 // ----------------------------------------------------------------------------