Initialize wxGenericProgressDialog button members in Init() too.

Initialize wxGenericProgressDialog button members in Init() too.

wxGenericProgressDialog::m_btn{Abort,Skip} were not initialized neither which
resulted in crashes inside EnableAbort() which was called if the "Cancel"
button was pressed in a native MSW dialog but the dialog wasn't cancelled (and
hence the button needed to be reenabled).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65509 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775