Don't update scrollbars when the window is being destroyed in wxOSX.

Don't update scrollbars when the window is being destroyed in wxOSX.

Updating scrollbars for a window that is being destroyed anyhow is useless and
resulted in crashes in the htlbox sample because it used client-to-screen
coordinates conversion which asserted because the TLW was invalid any more and
this unexpected assert during window destruction led to a crash.

Simply don't do it at all to avoid the problem.

Closes #11776.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66031 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775