Fix race condition cleaning up munin CGI graphs

Avoids mails like these:

From:    Cron Daemon <root@menotti.debian.org>
To:      root@menotti.debian.org
Subject: Cron <www-data@menotti> find /var/lib/munin/cgi-tmp -mindepth 2 -type d -empty -delete

find: `/var/lib/munin/cgi-tmp/munin-cgi-graph/debian.org/bm-bl9.debian.org/iostat-month.png': No such file or directory
find: `/var/lib/munin/cgi-tmp/munin-cgi-graph/debian.org/bm-bl9.debian.org/df_inode-day.png': No such file or directory
find: `/var/lib/munin/cgi-tmp/munin-cgi-graph/debian.org/bm-bl9.debian.org/ps_exim4-month.png': No such file or directory
find: `/var/lib/munin/cgi-tmp/munin-cgi-graph/debian.org/bm-bl9.debian.org/exim_mailstats-month.png': No such file or directory
...