crypto: dh - allow for passing NULL to the ffdheXYZ(dh)s' ->set_secret()

Ephemeral key generation can be requested from any of the ffdheXYZ(dh)
variants' common ->set_secret() by passing it an (encoded) struct dh
with the key parameter being unset, i.e. with ->key_size == 0. As the
whole purpose of the ffdheXYZ(dh) templates is to fill in the group
parameters as appropriate, they expect ->p and ->g to be unset in any
input struct dh as well. This means that a user would have to encode an
all-zeroes struct dh instance via crypto_dh_encode_key() when requesting
ephemeral key generation from a ffdheXYZ(dh) instance, which is kind of
pointless.

Make dh_safe_prime_set_secret() to decode a struct dh from the supplied
buffer only if the latter is non-NULL and initialize it with all zeroes
otherwise.

That is, it is now possible to call

  crypto_kpp_set_secret(tfm, NULL, 0);

on any ffdheXYZ(dh) tfm for requesting ephemeral key generation.

Signed-off-by: Nicolai Stange <nstange@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>