On svn-auth-x509 branch, fix a segfault with iconv is unavailable.

* subversion/libsvn_subr/utf.c
  (convert_string): Properly return the error if any.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/svn-auth-x509@1612966 13f79535-47bb-0310-9956-ffa450edef68