[Bug 281938] Make sure max_len is not 0 before using it as modulo

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=281938

--- Comment #7 from Michael Tuexen <tuexen@freebsd.org> ---
(In reply to nakayamakenjiro from comment #0)
If we are in the code path you mention, we are in the body of

        if (len + optlen + ipoptlen > tp->t_maxseg) {
                flags &= ~TH_FIN;

                if (tso) {
                        /* somewhere here */
                }
        }

tso is set by

        if ((tp->t_flags & TF_TSO) && V_tcp_do_tso && len > tp->t_maxseg &&
            (tp->t_port == 0) &&
            ((tp->t_flags & TF_SIGNATURE) == 0) &&
            (!sack_rxmit || V_tcp_sack_tso) &&
            (ipoptlen == 0 || (ipoptlen == ipsec_optlen &&
             (tp->t_flags2 & TF2_IPSEC_TSO) != 0)) &&
            !(flags & TH_SYN))
                tso = 1;

This means that ipoptlen == ipsec_optlen in all cases (if ipoptlen == 0, then
also ipsec_optlen == 0). Therefore tp->t_maxseg >= len + optlen + ipoptlen
holds due to the check in the if condition and finally tp->t_maxseg >= len +
optlen + ipsec_optlen holds, since ipoptlen == ipsec_optlen.
Therefore max_len = tp->t_maxseg - optlen - ipsec_optlen is positive.

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