Fix another instance of issue #3014: "svn log | head" should not print
"Write error: Broken pipe". This time, it affects help on subcommands.

$ svn help up | head
subversion/libsvn_subr/opt.c:317,
subversion/libsvn_subr/cmdline.c:446: (apr_err=SVN_ERR_IO_PIPE_WRITE_ERROR)
svn: E135007: Write error in pipe

* subversion/libsvn_subr/opt.c
  (svn_opt_subcommand_help3): Don't print anything for pipe-write errors.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1596866 13f79535-47bb-0310-9956-ffa450edef68