fix interface attribute exception #720 (thanks andr3w77)
fix interface attribute exception #720 (thanks andr3w77)

If an exception is thrown during the machinery.start() the interface
variable would not be set and thus cause an exception during the call to
unroute_network().

cuckoobox/cuckoo,cuckoobox/cuckoo,cuckoobox/cuckoo,cuckoobox/cuckoo,cuckoobox/cuckoo