Check if PG exists before adding or deleting a choice to wxPGProperty.

Because choice value can be added/deleted also to/from detached property object there is necessary to skip in wxPGProperty::InsertChoice and wxPGProperty::DeleteChoice the operations which are valid exclusively for property attached to the property grid.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76969 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775