findalldevstest: fix SIGSEGV (Segmentation fault) on Travis (Linux build)

findalldevstest: fix SIGSEGV (Segmentation fault) on Travis (Linux build)

This occurs with 'venet0' interface.
The reason of the NULL pointer is currently unkown.
There is now a message:
    Warning: a->addr is NULL, skipping this address.

Context:
Linux: 2.6.32 x86_64

$ ip link
2: venet0: <BROADCAST,POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN
    link/void

gdb trace:
--------------------------------------------------------------------------
venet0
        Loopback: no

Program received signal SIGSEGV, Segmentation fault.
0x0000000000403430 in ifprint (d=0x63e070) at ./tests/findalldevstest.c:69
69 switch(a->addr->sa_family)
--------------------------------------------------------------------------