Fix nil pointer dereference for tunnel device

Fix for a nil de-reference in FindAllDevs() for tunnel devices such as this one:
```
tun0      Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00  
          inet addr:46.246.47.79  P-t-P:46.246.47.79  Mask:255.255.255.0
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:3454 errors:0 dropped:0 overruns:0 frame:0
          TX packets:61188 errors:0 dropped:56249 overruns:0 carrier:0
          collisions:0 txqueuelen:100 
          RX bytes:1495075 (1.4 MiB)  TX bytes:86419971 (82.4 MiB)
```