Fix possible use of dangling window pointer in wxSizer::Insert()

Add the sizer item to the sizers items list only after calling of
wxWindowBase::SetContainingSizer() because it can throw (if it asserts
and the assert handler throws an exception, as happens in our own unit
tests) and then the sizer item would be kept in the sizers items list
but m_containingSizer wouldn't be set for the window.

src/common/sizer.cpp

@@ -673,7 +673,35 @@ wxSizer::~wxSizer()
 
 wxSizerItem* wxSizer::DoInsert( size_t index, wxSizerItem *item )
 {
-    m_children.Insert( index, item );
+    // The helper class that solves two problems when
+    // wxWindowBase::SetContainingSizer() throws:
+    // 1. Avoid leaking memory using the scoped pointer to the sizer item.
+    // 2. Disassociate the window from the sizer item to not reset the
+    //    containing sizer for the window in the items destructor.
+    class ContainingSizerGuard
+    {
+    public:
+        explicit ContainingSizerGuard( wxSizerItem *item )
+            : m_item(item)
+        {
+        }
+
+        ~ContainingSizerGuard()
+        {
+            if ( m_item )
+                m_item->DetachWindow();
+        }
+
+        wxSizerItem* Release()
+        {
+            return m_item.release();
+        }
+
+    private:
+        wxScopedPtr<wxSizerItem> m_item;
+    };
+
+    ContainingSizerGuard guard( item );
 
     if ( item->GetWindow() )
         item->GetWindow()->SetContainingSizer( this );
@@ -681,7 +709,9 @@ wxSizerItem* wxSizer::DoInsert( size_t index, wxSizerItem *item )
     if ( item->GetSizer() )
         item->GetSizer()->SetContainingWindow( m_containingWindow );
 
-    return item;
+    m_children.Insert( index, item );
+
+    return guard.Release();
 }
 
 void wxSizer::SetContainingWindow(wxWindow *win)