Add more nd_print_trunc() calls

[tcpdump] / tests / icmp6_mobileprefix_asan.out

IP6 (class 0x50, flowlabel 0x0002c, hlim 0, next-header ICMPv6 (58) payload length: 7168) 4f:f829:c:1a1a:1a1a:1a1a:1a37:0 > 16:0:400:0:64fb:9303:f293:8200: ICMP6, mobile router advertisement, length 7168, id 0x9393[|icmp6]
[Invalid header: caplen==0, len==0]