Changes

core: make sure that cousin pointers are sane after merging root with
an identical child
When merging identical parent/child, the root may be replaced with a
child. Make sure we clear old cousin pointers in that child. Usually
they are cleared when rebuilding levels, but the root level is never
rebuilt.
This fixes a debug assert on a dual-PU Solaris machines where the
second PU is offline.
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>