Consistently use 4, rather than a sizeof, as the size of an IPv4 address.

[tcpdump] / tests / isis-seg-fault-3-v.out

IS-IS, length 131146
        L2 LSP, hlen: 27, v: 1, pdu-v: 1, sys-id-len: 6 (0), max-area: 3 (0)
          lsp-id: 1111.1111.1111.00-00, seq: 0x00000007, lifetime:  1200s
          chksum: 0x378e [|isis]