Don't generate drag events without valid item in wxGenericListCtrl.

Don't generate drag events without valid item in wxGenericListCtrl.

Dragging the mouse from the empty area of the control must not generate any
drag events as there is nothing to drag there. Also, MSW version doesn't do
it.

So fix the generic version to avoid generating such invalid drag events too by
checking that we have a valid item before checking that we're dragging.

Closes #1619.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70296 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775