safety and tidy the port_macs patch

lib/App/Netdisco/Util/PortMAC.pm

@@ -29,9 +29,9 @@ addresses supplied as array reference
 =cut
 
 sub get_port_macs {
-
     my ($fw_mac_list) = @_;
     my $port_macs = {};
+    return {} unless defined $fw_mac_list and ref [] eq ref $fw_mac_list;
 
     my $dp_macs
         = schema('netdisco')->resultset('DevicePort')

lib/App/Netdisco/Worker/Plugin/Macsuck/Nodes.pm

@@ -278,8 +278,7 @@ sub walk_fwtable {
     ? {} : $snmp->qb_fw_vlan;
   my $bp_index = $snmp->bp_index;
 
-  my @fw_mac_list = values %$fw_mac;
-  my $port_macs = get_port_macs(\@fw_mac_list);
+  my $port_macs = get_port_macs( values %$fw_mac );
 
   # to map forwarding table port to device port we have
   #   fw_port -> bp_index -> interfaces