util: clear pointers on wl_list_remove()
util: clear pointers on wl_list_remove()

Set the next and prev pointers of the removed list element to NULL. This
will catch programming errors that would use invalid list pointers,
double-remove for instance.

It also helps debugging, making it easy to see in gdb if an object is
not in a list.

Signed-off-by: Pekka Paalanen <6c0263d81461c956d504307112a4ce25eaacc3d1@gmail.com>

tulcod/wayland,tulcod/wayland,abooij/wayland,eyolfson/wayland,mchalupa/wayland,tulcod/wayland,bitfactor/wayland,mchalupa/wayland,sir-murray/wayland,bitfactor/wayland,sir-murray/wayland,eyolfson/wayland,abooij/wayland,mchalupa/wayland,sir-murray/wayland,eyolfson/wayland,bitfactor/wayland,abooij/wayland