little-endian constructor ignoring important zeroes

I noticed that constructor is ignoring important zeroes on little-endian constructor.
Example:
```js
x = new BN('0000ff', 16, 'le')
<BN: ff>   // should be  <BN: ff0000>
x = new BN('010000ff', 16, 'le')
<BN: ff000001>
```