Avoid crashes in wxMSW when using buttons without valid parent.

Avoid crashes in wxMSW when using buttons without valid parent.

Child controls such as buttons are supposed to always have a valid parent but
if, somehow, they don't, still don't crash in wxMSW button code but just
trigger an assert failure instead of unavailable button top level parent
instead.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70935 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775