BigInteger:  small optimization to bitwise_not

See https://github.com/phpseclib/phpseclib/pull/431#issuecomment-50237254
You're right. Rewriting it that way makes it more clear that the initial fix was wrong. eg.

```
$temp = str_pad($temp, strlen($temp), chr(0), STR_PAD_LEFT);
```

That's obviously not going to be doing any padding.

The thing is, `$leading_bits` and `$temp` need to be of the same length for the `$leading_ones | $temp` to work correctly. `$this->_base256_lshift($leading_ones, $current_bits);` changes the length of `$leading_ones`. When I was last working on that function I probably saw the `$this` of `$this->_base256_lshift` and did `$this->toBits()` in response thinking that `$this` was what was being changed instead of `$leading_ones`, which is actually the one being changed.