Fix bugs related to two phase creation of wxRibbon classes.

Add missing wxRibbonControl::Create() method. Ensure that member variables are
always initialized by the ctor. Check that we're fully initialized in EVT_SIZE
handler.

Closes #12018.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64243 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/ribbon/panel.cpp

@@ -43,7 +43,7 @@ BEGIN_EVENT_TABLE(wxRibbonPanel, wxRibbonControl)
     EVT_SIZE(wxRibbonPanel::OnSize)
 END_EVENT_TABLE()
 
-wxRibbonPanel::wxRibbonPanel()
+wxRibbonPanel::wxRibbonPanel() : m_expanded_dummy(NULL), m_expanded_panel(NULL)
 {
 }