dp_packet_at(const struct dp_packet *b, size_t offset, size_t size)
{
    return offset + size <= dp_packet_size(b)
           ? (char *) dp_packet_data(b) + offset
           : NULL;
}
dp_packet_at(const struct dp_packet *b, size_t offset, size_t size)
{
    return offset + size <= dp_packet_size(b)
           ? (char *) dp_packet_data(b) + offset
           : NULL;
}