Make the byteLength work correctly when input is Buffer.
e.g:
```js
// The incomplete unicode string
Buffer.byteLength(new Buffer([0xe4, 0xb8, 0xad, 0xe6, 0x96]))
```
The old output: 9
The new output: 5
PR-URL: https://github.com/nodejs/node/pull/4738
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Jackson Tian
8d0ca10752