Fix compiler warning caused by using wrong type.

* subversion/libsvn_subr/x509parse.c
  (is_hostname): Use apr_size_t for the iterator since svn_string_t's len
    is an apr_size_t.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/svn-auth-x509@1616634 13f79535-47bb-0310-9956-ffa450edef68