Fix NULL deference when validating FFC public key.

Fix NULL deference when validating FFC public key.

Fixes CVE-2023-0217

When attempting to do a BN_Copy of params->p there was no NULL check.
Since BN_copy does not check for NULL this is a NULL reference.

As an aside BN_cmp() does do a NULL check, so there are other checks
that fail because a NULL is passed. A more general check for NULL params
has been added for both FFC public and private key validation instead.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>