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: 4495
Signed-off-by: Werner Koch <wk@gnupg.org>
(cherry picked from 2.2 commit 8d0d61aca3)

sm/certdump.c

@@ -658,6 +658,10 @@ format_name_writer (void *cookie, const void *buffer, size_t size)
   struct format_name_cookie *c = cookie;
   char *p;
 
+  /* FIXME: Replace the whole thing using es_fopenmem based code.  */
+  if (!buffer)  /* Flush. */
+    return 0;  /* (Actually we could use SIZE because that should be 0 too.)  */
+
   if (!c->buffer)
     {
       p = xtrymalloc (size + 1 + 1);