Fix a ra_serf crash found by running svnrdump against a NTLM enabled repository.
Fix a ra_serf crash found by running svnrdump against a NTLM enabled
repository.

* subversion/libsvn_ra_serf/win32_auth_sspi.c
  (do_auth):
    Don't segfault on a NULL auth_attr, as that is a valid value when the
    nogatiation hasn't started yet.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion