Keep a buffer written by WritePointer alive
until the finalizers for the buffer to which
the pointer has been run have been executed:

Refs: node-ffi-napi#54

Signed-off-by: Michael Dawson <mdawson@devrus.com>

Update to avoid changing behaviour for
_writePointer and limit change to writePointer
where objects were already being associated
with the buffer.

Signed-off-by: Michael Dawson <mdawson@devrus.com>

remove _attach call in writePointer as
it is no longer needed since a stronger
version is done in the native code
when true is passed as the fourth parameter