Hardware.pm errors when swap is not enabled 

When swap is checked (e.g. with `dump_system_information()` but a system does not have swap enabled, `Use of uninitialized value` errors are also dumped to the console. 

Reporting from a FreeBSD-9 system  - but I think this would occur on other platforms as well.

See issue https://github.com/RexOps/Rex/issues/452

This is just a quick (FreeBSD only) patch for now. Obviously a more general approach is needed: perhaps where `defined`ness checks are done outside of each per OS case?  I'm not sure how to approach this but will watch and learn :-)

Cheers and thanks for Rex.