return NULL (and not invalid) iterator from compatibility_iterator::GetPrevious() when we're at the start of the list (see patch 1445506)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@38243 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/list.h

@@ -190,6 +190,9 @@ private:
             }                                                                 \
             compatibility_iterator GetPrevious() const                        \
             {                                                                 \
+                if ( m_iter == m_list->begin() )                              \
+                    return compatibility_iterator();                          \
+                                                                              \
                 iterator i = m_iter;                                          \
                 return compatibility_iterator( m_list, --i );                 \
             }                                                                 \