utf16leSlice behaviour differs from node.js

Problem
```js
const oddLengthBuffer = Buffer.from('abcde', 'utf8')
console.log(Buffer.from(oddLengthBuffer.toString('utf16le'), 'utf16le').toString('utf8')) // ‘abcd’ in Node.js and ‘abcd\x00\x00’ with polyfill
```

This comes from accessing n+1-th element of a Buffer of size n in utf16leSlice.
Node.js simply omits it.

Proposed solution: https://github.com/feross/buffer/pull/250

