Fix case when current-prefix is list

current-prefix is list such as 'C-u M-x helm-swoop'.
helm-swoop-last-prefix-number is set it, but this causes error.
helm-swoop-last-prefix-number should be integer.

I got following error when I execute `C-u M-x helm-swoop`.
```
if: Wrong type argument: number-or-marker-p, (4)
```