buffer: fix range checking for slowToString

If `start` is not a valid number in the range, then the default value
zero will be used. Same way, if `end` is not a valid number in the
accepted range, then, by default, the length of the buffer is assumed.

Fixes: https://github.com/nodejs/node/issues/2668
Ref: https://github.com/nodejs/node/pull/2919
PR-URL: https://github.com/nodejs/node/pull/4019


Reviewed-By: Trevor Norris <trev.norris@gmail.com>