fix message about the amount of addresses changed

[brunoerg]

In the case we have 0 IPv4 and/or IPv6 addresses changed when comparing two files, the following line would throw an error (math.log2(0) doesn't make sense):

print("# %i (2^%f) IPv4 addresses changed; %i (2^%f) IPv6 addresses changed" % (ipv4_changed, math.log2(ipv4_changed), ipv6_changed, math.log2(ipv6_changed)))

[dunxen]

ACK 028a29c