RT4002: check for NULL cipher in p12_crpt.c

The NULL cipher case can't actually happen because we have no
EVP_PBE_CTL combinations where cipher_nid is -1 and keygen is
PKCS12_PBE_keyivgen. But make the code more obviously correct.

Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit 394f7b6f)