Fix invalid malloc failures in PEM_write_bio_PKCS8PrivateKey()

Fix invalid malloc failures in PEM_write_bio_PKCS8PrivateKey()

When `PEM_write_bio_PKCS8PrivateKey()` was passed an empty passphrase
string, `OPENSSL_memdup()` was incorrectly getting used for 0 bytes size
allocation, which resulted in malloc failures.

Fixes: https://github.com/openssl/openssl/issues/17506
Signed-off-by: Darshan Sen <raisinten@gmail.com>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17507)