On the svn-auth-x509 branch, add missing if test.

* subversion/libsvn_subr/x509parse.c
  (x509_get_uid): This block was only intended to run when there was an error.
    Prior to this it would segfault on any cert that actually did have v2/v3
    unique identifiers.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/svn-auth-x509@1605643 13f79535-47bb-0310-9956-ffa450edef68