Check for existance of cookie before using.

Fixes instances where we try to get a value from a cookie which might
not have been set yet.

Clears up the following errors in the logs

```
[Tue Sep 02 18:02:11 2014] [error] [client 127.0.0.1] PHP Notice:  Undefined index: nagdash_unwanted_hosts in /var/www/Nagdash/htdocs/nagdash.php on line 24, referer: http://localhost/
[Tue Sep 02 18:02:11 2014] [error] [client 127.0.0.1] PHP Notice:  Undefined index: nagdash_hostfilter in /var/www/Nagdash/htdocs/nagdash.php on line 27, referer: http://localhost/
```