crypto: dh - Remove pointless checks for NULL 'p' and 'g'

Neither 'p' nor 'g' can be NULL, as they were unpacked using
crypto_dh_decode_key().  And it makes no sense for them to be optional.
So remove the NULL checks that were copy-and-pasted into both modules.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>