x509/x509name.c: fix potential crash in X509_NAME_get_text_by_OBJ.

Documentation says "at most B<len> bytes will be written", which
formally doesn't prohibit zero. But if zero B<len> was passed, the
call to memcpy was bound to crash.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6860)