Avoid an assert in SVN_DEBUG mode for svn_error_dup(SVN_NO_ERROR).

* subversion/libsvn_subr/error.c
  (svn_error_dup): Simply return for SVN_NO_ERROR.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1583997 13f79535-47bb-0310-9956-ffa450edef68