[krbdev.mit.edu #2654] krb5_get_init_creds() should be aware of 32 bit min and max for times

[Alexandra Ellwood via RT]

krb5_get_init_creds() should be aware of 32 bit minimum and maximum values for time 
values such as start time, end time and renew until time.  Currently it performs addition on 
these values without checking for overflow.  This results in unexpected behavior if the user 
attempts to request tickets which expire after 2038.