Fixed ipaddresses function in Puppet 3.x

The ipaddresses function is expecting lookupvar to return :undefined if
a value is not set.  That was the correct behavior in Puppet 2.7.  As of
Puppet 3.0, it returns nil instead.

See http://docs.puppetlabs.com/guides/custom_functions.html
Merge pull request #40 from Element84/HandleMissingInterfaces

Fixed ipaddresses function in Puppet 3.x