Try to detect invalid wxHtmlWindow title format earlier.

Assert directly in wxHtmlWindow::SetRelatedFrame() instead of doing it much
later, hopefully people will read the comment at the assert point.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75748 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775