buffer: fix check for `.buffer` property

isSharedArrayBuffer in fromObject was missing obj.buffer
moved the 'length' in obj check so that it is checked first making
the code slightly more performant and able to handle SharedArrayBuffer
without relying on an explicit check.

Ref: https://github.com/nodejs/node/pull/8510
PR-URL: https://github.com/nodejs/node/pull/8739


Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>