unbuffer stdin before get passwd from stdin

unbuffer stdin before get passwd from stdin

commond LD_LIBRARY_PATH= openssl rsa -aes256 -passout stdin <<< "xxxxxx” will get pass(fun app_get_pass()) from stdin first, and then load key(fun load_key()). but it unbuffer stdin before load key, this will cause the load key to fail.

now unbuffer stdin before get pass, this will solve https://github.com/openssl/openssl/issues/19835

CLA: trivial

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19851)