crypto: shash - Allow cloning on algorithms with no init_tfm

Some shash algorithms are so simple that they don't have an init_tfm
function.  These can be cloned trivially.  Check this before failing
in crypto_clone_shash.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>