Since commit 14b0a83f64 we store the
main window monitor instead of computing it every time. This means
that we must now ensure that it's updated before trying to use it
which we do from meta_screen_resize_func() or else we'll crash on an
assertion later on when removing a monitor:
assertion failed: (which_monitor < workspace->screen->n_monitor_infos)
https://bugzilla.gnome.org/show_bug.cgi?id=752674
7e1c6ff2a2