filter_if: handle hwaddr not existing

couldn't handle hwaddr element not existing, patched.

```
(dev@192.168.1.254)8> inet:getifaddrs().
{ok,[{"lo0",
      [{flags,[up,loopback,running,multicast]},
       {addr,{65152,0,0,0,0,0,0,1}},
       {netmask,{65535,65535,65535,65535,0,0,0,0}},
       {addr,{127,0,0,1}},
       {netmask,{255,0,0,0}},
       {addr,{0,0,0,0,0,0,0,1}},
       {netmask,{65535,65535,65535,65535,65535,65535,65535,
                 65535}}]},
     {"gif0",[{flags,[pointtopoint,multicast]}]},
     {"stf0",[{flags,[]}]},
     {"en0",
      [{flags,[up,broadcast,running,multicast]},
       {hwaddr,[60,7,84,49,199,123]},
       {addr,{65152,0,0,0,15879,21759,65073,51067}},
       {netmask,{65535,65535,65535,65535,0,0,0,0}},
       {addr,{192,168,1,254}},
       {netmask,{255,255,255,0}},
       {broadaddr,{192,168,1,255}}]},
     {"en1",
      [{flags,[up,broadcast,running,multicast]},
       {hwaddr,[104,168,109,27,250,132]}]},
     {"p2p0",
      [{flags,[broadcast,multicast]},
       {hwaddr,[10,168,109,27,250,132]}]},
     {"fw0",
      [{flags,[up,broadcast,running,multicast]},
       {hwaddr,[164,177,151,255,254,104,48,212]}]},
     {"vnic0",
      [{flags,[up,broadcast,running,multicast]},
       {hwaddr,[0,28,66,0,0,8]},
       {addr,{10,211,55,2}},
       {netmask,{255,255,255,0}},
       {broadaddr,{10,211,55,255}},
       {addr,{65152,0,0,0,540,17151,65024,8}},
       {netmask,{65535,65535,65535,65535,0,0,0,0}},
       {addr,{65216,0,0,65193,0,0,0,...}},
       {netmask,{65535,65535,65535,65535,0,0,...}}]},
     {"vnic1",
      [{flags,[up,broadcast,running,multicast]},
       {hwaddr,[0,28,66,0,0,9]},
       {addr,{10,37,129,2}},
       {netmask,{255,255,255,0}},
       {broadaddr,{10,37,129,255}},
       {addr,{65152,0,0,0,540,17151,65024,9}},
       {netmask,{65535,65535,65535,65535,0,0,0,...}},
       {addr,{65216,0,0,65194,0,0,...}},
       {netmask,{65535,65535,65535,65535,0,...}}]},
     {"jnc0",
      [{flags,[up,running]},
       {addr,{10,172,10,43}},
       {netmask,{255,255,255,255}}]},
     {"jnc1",[{flags,[up,running]}]}]}
```