On svn-auth-x509 branch, don't crash on v1/2 certs.

* subversion/libsvn_subr/x509parse.c
  (x509parse_get_hostnames): Check for NULL, since v1/2 certs don't have
    extensions.

Reported by: philip


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/svn-auth-x509@1619861 13f79535-47bb-0310-9956-ffa450edef68