mesa: deal with vbo_save_vertex_list::modes being NULL
What does this MR do and why?
mesa: deal with vbo_save_vertex_list::modes being NULL
This case was forgotten in the 9aa205668bc.
Fixes: 9aa205668bc ("mesa: inline {Create,Draw}GalliumVertexState callbacks")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/10619