SMIME_text() and SMIME_crlf_copy() small refactoring

SMIME_text() and SMIME_crlf_copy() small refactoring

When out is NULL, SMIME_text() now skips BIO_write entirely and continues
to return success based on the read loop result. This matches existing
and expected semantics while avoiding a pointless write and any error masking.

Signed-off-by: Joshua Rogers <MegaManSec@users.noreply.github.com>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28879)