ntpd NTS client denial of service via wrongly sized cookies

Summary

Two denial of service vulnerabilities were found in ntpd-rs related to the handling of NTS cookies in our client functionality. Whenever an NTS source is configured and the server behind that source is sending zero-sized cookies or cookies larger than what would fit in our buffer size, ntpd-rs would crash. Only configured NTS sources can abuse these vulnerabilities. NTP sources or third parties that are not configured cannot make use of these vulnerabilities. For zero-sized cookies: a division by zero would force an exit when the number of new cookies that would need to be requested is calculated. In ntpd-rs 1.5.0 a check was added to prevent the division by zero. For large cookies: while trying to send a NTP request with the cookie included, the buffer is too small to handle the cookie and an exit of ntpd-rs is forced once a write to the buffer is attempted. The memory outside the buffer would not be written to in this case. In ntpd-rs 1.5.0 a check was added that prevents accepting cookies larger than 350 bytes. Users of older versions of ntpd-rs are recommended to update to the latest version. If an update is impossible, it is recommended to only add NTS sources to ntpd-rs that are trusted to not abuse this bug.