buffer: fix map and set parent to undefined

In 4c9b30d removal of the prototype attributes meant NativeBuffer() no
longer had the same object map as Buffer(). By now setting the same
properties in the same order both constructors will produce the same
map.

The same commit changed "parent" from undefined to null. This caused a
failure in Buffer#slice() where it was checked if parent === undefined.
Causing the incorrect parent to be set.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>