Update macaddress.rb

First pull request, so bear with me...

Before this change, 'path' didn't exist so it would always return 'nil' when hitting this resolution.

For some reason it isn't falling back to the other resolution (at least on SLES 11 SP3), but that may be a separate issues.

Also didn't think we needed to check for the existence of path in the second part since it would hit the existing exception block if it didn't exist.