Allow setting icon before m_widget is valid.
There is no need to require a valid m_widget,
the "realize" handler will take care of things later.
Closes #16731


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@78270 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775