libnetutil: Check dhcp respose packet length

     * to construct the pseudo header used in the checksum calculation.

     */

    dhcp_size = ntohs(packet.udp.len) - sizeof(packet.udp);

    /*

     * check validity of dhcp_size.

     * 1) cannot be negative or zero.

     * 2) src buffer contains enough bytes to copy

     * 3) cannot exceed destination buffer

     */

    if ((dhcp_size <= 0) ||

        ((int)(nread - sizeof(struct iphdr) - sizeof(struct udphdr)) < dhcp_size) ||

        ((int)sizeof(struct dhcp_msg) < dhcp_size)) {

#if VERBOSE

        ALOGD("Malformed Packet");

#endif

        return -1;

    }

    saddr = packet.ip.saddr;

    daddr = packet.ip.daddr;

    nread = ntohs(packet.ip.tot_len);