Fix wxWindow::IsDescendant() to work with argument equal to this window.

Fix wxWindow::IsDescendant() to work with argument equal to this window.

Passing the window itself as IsDescendant() argument for a top level window
resulted in a NULL pointer dereference. Fix this and also simplify the
function code by not using the parent window before checking it's !NULL.

Closes #14387.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71702 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775