sm: Fix a warning in an es_fopencooie function.

* sm/certdump.c (format_name_writer): Take care of a flush request.

We won't see a flush here so that fix does not solve a real bug but we
want to be correct. Note that this function seems to predate the
es_fopenmem function and thus in master (2.3) we should replace it
entirely.

• GnuPG-bug-id: T4495
• Signed-off-by: Werner Koch <wk@gnupg.org>