Do more bounds checking and length checking.

[tcpdump] / tests / dio.out

IP6 (hlim 255, next-header ICMPv6 (58) payload length: 24) fe80::1000:ff:fe64:6423 > ff02::1: [icmp6 sum ok] ICMP6, RPL, length 24, DAG Information Object [seq:10,instance:42,rank:1,dagid:thisismynicedag1]