Return an invalid item from wxDataViewCtrl::GetItemByRow() for invalid rows.

Return an invalid item from wxDataViewCtrl::GetItemByRow() for invalid rows.

This function is called in many places in the code with possibly invalid (i.e.
out of range) row, so handle it gracefully inside it in virtual list control
case. This is consistent with the behaviour in non-virtual case and with that
of GetRowByItem().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70717 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775