pki_crypto: Fix private key generation with password

We need to specify a cipher when we generate a key with a password.
OpenSSH uses aes_128_cbc, so we should use the same.

Thanks to Julian Lunz for the report.

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>

tests/unittests/torture_pki_ecdsa.c

@@ -354,7 +354,7 @@ static void torture_pki_ecdsa_write_privkey(void **state)
     unlink(LIBSSH_ECDSA_TESTKEY);
 
     rc = ssh_pki_export_privkey_file(origkey,
-                                     "",
+                                     NULL,
                                      NULL,
                                      NULL,
                                      LIBSSH_ECDSA_TESTKEY);