treebox::clear failed repair and arg_dropfiles::pos repair

[uuiid]

The incoming node may be directly a pointer to the root node 传入节点有可能直接是根节点的指针

[uuiid]

::DragQueryPoint(drop, &mswin_pos); Returns the location of the point dragged into the file window Is the position relative to the registered window handle https://docs.microsoft.com/en-us/windows/win32/api/shellapi/nf-shellapi-dragquerypoint

[cnjinhao]

Thanks for your PR. It has been merged into branch develop-1.8