fix slh-dsa incorrect prediction of result code

fix slh-dsa incorrect prediction of result code

The slh_dsa fuzzer predicts failure in EVP_message_sign_init in the
event we pass a context_string param of more than 255 bytes.  That makes
for an accurate prediction, but only if we actually create  the param.

augment the setting of exepct_rc_init to be determined not only by our
allocation of a > 255 byte message, but also on selector bit 1, which
determines if we create the parameter at all.

Fixes https://oss-fuzz.com/testcase-detail/4807793999937536

Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/26884)