Monthly cron generates error if wtmp hasn't been rotated

[Rowan Wookey]

The logrotate config for wtmp on focal looks like this:

# no packages own wtmp -- we'll rotate it here
/var/log/wtmp {
    missingok
    monthly
    create 0664 root utmp
    minsize 1M
    rotate 1
}

It only rotates if wtmp is larger than 1M. In the case of a fresh install that has a wtmp smaller than 1M it won't rotate and /var/log/wtmp.1 won't exist, in this case we should use /var/log/wtmp

I've attached a patch.