stream.decode(cb) throws Error: Data too short

Nobody has claimed this yet.

When calling `stream.decode()` on the `moov` box, the mp4-box-encoding module throws an error:

```
node_modules/mp4-stream/node_modules/mp4-box-encoding/index.js:139
throw new Error('Data too short')
^

Error: Data too short
at Object.Box.decode (node_modules/mp4-stream/node_modules/mp4-box-encoding/index.js:139:11)
at exports.AudioSampleEntry.decode (node_modules/mp4-stream/node_modules/mp4-box-encoding/boxes.js:374:21)
at Object.Box.decodeWithoutHeaders (node_modules/mp4-stream/node_modules/mp4-box-encoding/index.js:169:11)
at Object.Box.decode (node_modules/mp4-stream/node_modules/mp4-box-encoding/index.js:142:14)
at exports.stsd.decode (node_modules/mp4-stream/node_modules/mp4-box-encoding/boxes.js:241:21)
at Object.Box.decodeWithoutHeaders (node_modules/mp4-stream/node_modules/mp4-box-encoding/index.js:169:11)
at Object.Box.decode (node_modules/mp4-stream/node_modules/mp4-box-encoding/index.js:142:14)
at Object.Box.decodeWithoutHeaders (node_modules/mp4-stream/node_modules/mp4-box-encoding/index.js:155:23)
at Object.Box.decode (node_modules/mp4-stream/node_modules/mp4-box-encoding/index.js:142:14)
at Object.Box.decodeWithoutHeaders (node_modules/mp4-stream/node_modules/mp4-box-encoding/index.js:155:23)
```

This is the code:

``` js
var fs = require('fs')
var mp4 = require('mp4-stream')

var stream = mp4.decode()

stream.on('box', function (headers) {
if (headers.type === 'moov') stream.decode(function () {})
else stream.ignore()
})

fs.createReadStream('./test.m4v').pipe(stream)
```

Reproduce the failure with test.m4v using the shown mp4-stream decoder and inspect the moov, stsd, and AudioSampleEntry decode paths in mp4-box-encoding/index.js and boxes.js. Trace why stream.decode reaches the Data too short error, then verify that decoding the same moov completes without that error.

Written by the indexing model from the issue text.