Fix fatal bug when changing wxDataViewCtrl model in wxGTK.

Fix fatal bug when changing wxDataViewCtrl model in wxGTK.

wxDataViewCtrl::AssociateModel() wrongly delete m_notifier itself if the
control already had a model even though the notifier was registered using
AddNotifier() which also scheduled it for deletion when the old model itself
was destroyed. This resulted in double deletion and reproducible crash
whenever the model was changed.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62577 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775