(for 4.9.3) CVE-2018-14470/Babel: fix an existing length check

[tcpdump] / tests / pim_header_asan.out

IP6 (class 0x76, flowlabel 0xf6767, hlim 109, next-header PIM (103) payload length: 30311) 6767:6767:6767:8267:6767:6765:6767:6767 > 6700:80:74:24:2424:2424:2424:2509: PIMv2, length 30311
        Bootstrap, RFC2117-encoding[|pim]