sexp: String with \0 is considered "binary".

* src/sexp.c (suitable_encoding): It's "binary" when
the buffer contains '\0'.

Before the fix, it hits a corner case of strchr returning non-NULL
with *s = '\0', resulting shorter representation with "...". See
strchr(3).

• GnuPG-bug-id: T6747
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>