Bug#454989: check if really will use the lock

[jidanni@jidanni.org]

Besides the patch below, it is very important that you first make sure
that you really intend to access the cache: check all the variables
first.

Otherwise if the cron job fires when apt is in use, it will send an
error message to root. Which happens about once a month for me.

*** /etc/cron.daily/apt	2007-10-27 04:27:00.000000000 +0800
--- /tmp/apt	2008-01-09 09:24:06.875768181 +0800
***************
*** 172,177 ****
--- 172,178 ----
  
  # check if we can lock the cache and if the cache is clean
  if ! apt-get check -q -q 2>/dev/null; then
+     echo Cannot lock the cache, exiting.
      exit 1
  fi