Don't start dragging invalid items in generic wxDataViewCtrl

It's not useful to allow dragging from the control area where there are
no items and it resulted in assert failures due to using an invalid row
in wxDataViewDropSource::GiveFeedback(), so simply don't do it at all.

Note that native GTK wxDataViewCtrl implementation doesn't do this
neither, so it also makes the behaviour more consistent.

Closes https://github.com/wxWidgets/wxWidgets/pull/1828

src/generic/datavgen.cpp

@@ -4703,6 +4703,9 @@ void wxDataViewMainWindow::OnMouse( wxMouseEvent &event )
             m_owner->CalcUnscrolledPosition( m_dragStart.x, m_dragStart.y,
                                              &m_dragStart.x, &m_dragStart.y );
             unsigned int drag_item_row = GetLineAt( m_dragStart.y );
+            if (drag_item_row >= GetRowCount() || m_dragStart.x > GetEndOfLastCol())
+                return;
+
             wxDataViewItem itemDragged = GetItemByRow( drag_item_row );
 
             // Notify cell about drag