negotiate_kerberos_auth: better bounds checking

negotiate_kerberos_auth: better bounds checking

* sysconf() may return -N values on some platforms or values larger than
  the hard-coded 1024 buffer size for hostname. Use sizeof() instead
  since the buffer is hardcoded anyway.

* also, use return instead of exit() on the test binary to reduce
  warnings from static analysis compilers.

 Detected by Coverity Scan. Issues 740392, 740484