ssl/ssl_txt: fix NULL-check

NULL-check for cipher is redundant, instead check if cipher->name is NULL

While here fix formatting of BIO_printf calls as suggested by Andy Polyakov.

CLA: trivial

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6282)