Avoid buffer overrun with 'binary' encoding

[koichik]

With 'binary' encoding, Buffer.write() overruns the buffer's length.
reproduce:

> buf = new Buffer([0, 0, 0, 0, 0]);
<Buffer 00 00 00 00 00>
> sub = buf.slice(0, 4);
<Buffer 00 00 00 00>
> sub.write('12345', 'binary')
5
> buf
<Buffer 31 32 33 34 35>

Please review.

[ry]

LGTM

[koichik]

Thanks!