Fix for byte-compile warnings

I got following byte-compile warnings with origin code.

```
In coverlay-create-stats-buffer:
coverlay.el:74:27:Warning: `beginning-of-buffer' is for interactive use only;
    use `(goto-char (point-min))' instead.

In coverlay-overlay-current-buffer-with-list:
coverlay.el:195:51:Warning: `beginning-of-buffer' is for interactive use only;
    use `(goto-char (point-min))' instead.
```