Only return MAC from munge_mac() if it actually is a MAC.  Fix for netdisco where device would not be inserted in DB due to malformed MAC.
Only return MAC from munge_mac() if it actually is a MAC.  Fix for netdisco where device would not be inserted in DB due to malformed MAC.

lucwillems/SNMP-INFO,42wim/snmp-info,lucwillems/SNMP-INFO,42wim/snmp-info,lucwillems/SNMP-INFO