rounding 9.9 gives improper coefficient

When rounding `Big('9.9')`, the resulting Big object will be:
```
Big {
  s: 1,
  e: 1,
  c: [ 1, '-1': NaN ],
  constructor: [Function: Big] {
    DP: 20,
    RM: 1,
    NE: -7,
    PE: 21,
    strict: true,
    roundDown: 0,
    roundHalfUp: 1,
    roundHalfEven: 2,
    roundUp: 3,
    Big: [Circular],
    default: [Circular]
  }
}
```
The index -1 of c is NaN.