buffer: fix writeInt{B,L}E for some neg values

The algorithm used to convert negative values to hex generates incorrect
values when the low byte(s) of the value are zero because a carried
subtraction is applied prematurely.

Fixes: https://github.com/nodejs/node/issues/3992
PR-URL: https://github.com/nodejs/node/pull/3994


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Signed-off-by: Peter A. Bigot <pab@pabigot.com>