[krbdev.mit.edu #7553] sendto_kdc can invoke poll with negative timeout

[Greg Hudson via RT]

Our select/poll wrapper in cm.c and sendto_kdc.c uses an absolute end 
time.  If an event occurs just prior to the end time and doesn't terminate 
the service_fds loop, and we take a few milliseconds to process it, we 
could wind up calling cm_select_or_poll after the end time has expired.  
The select wrapper in cm.c is careful to check for this, but the poll 
wrapper in sendto_kdc.c is not.  If we invoke poll with a negative 
timeout, it will wait indefinitely.

This issue was reported in a github pull request by Matthieu Hautreax 
<matthieu.hautreux at gmail.com>.