RangeError in authCipher.js

Hi, thanks for provide this package for aes support. When I try aes encrypt this error comes out, but it seems I can not fix it from outside.

Error message:

```
RangeError [ERR_OUT_OF_RANGE]: The value of "byteLength" is out of range. It must be >= 1 and <= 6. Received 8
at boundsError (internal/buffer.js:55:9)
at Buffer.writeUIntBE (internal/buffer.js:588:3)
at webpack:///node_modules/browserify-aes/authCipher.js:37 <- index.js:7:174349
at new u (webpack:///node_modules/browserify-aes/authCipher.js:52 <- index.js:7:174434)
at Object.l (webpack:///node_modules/browserify-aes/encrypter.js:99 <- index.js:7:256315)
at f (webpack:///node_modules/browserify-cipher/browser.js:45 <- index.js:7:245646)
at Function.encrypt (webpack:///src/util/Crypto.js:82:34 <- index.js:7:237811)
at Context.it (webpack:///test/unit/specs/util/Crypto.spec.js:8:30 <- index.js:7:232580)
```

Error code:
```js
ghash.update(Buffer.alloc(8, 0))
var ivBits = len * 8
var tail = Buffer.alloc(8)
tail.writeUIntBE(ivBits, 0, 8) // <- here
ghash.update(tail)
```