Fix IPID sequence detection in the case of Random

Reported by Lior Levinsky. As part of r32469, which added IPv6 IPID
sequnce detection, the logic to detect all-zero IPID sequences was
split. get_diffs was returning IPID_SEQ_UNKNOWN, IPID_SEQ_RD, or
1 for all-zeros, but the get_ipid_sequence_* functions were treating
every non-zero return value as indicating all-zeros, which meant that
IPID sequence detection was broken.

http://seclists.org/nmap-dev/2014/q1/287