Overflow when checking for delay attacks

[sijohans]

From review:

https://github.com/assaabloy-ppi/salt-channel-c/blob/master/src/salti_util.c#L303

 if (t_arrival - p_channel->peer_epoch > t_package + p_channel->delay_threshold) {
    /* Delay detected */
    SALT_ERROR(SALT_ERR_DELAY_DETECTED);
}

Issues:

• This does not handle integer overflow nicely, left side may
  underflow, right side may overflow; behavior is unpredictable on
  extreme values.
• This check “larger than” condition instead of “is absolute difference
  larger than”, i.e. only one out of two type of clock drift and
  strange timing issues is detected.