crypto: stm32 - Fix empty message processing

Change the emptymsg check in stm32_hash_copy_hash to rely on whether
we have any existing hash state, rather than whether this particular
update request is empty.

Also avoid computing the hash for empty messages as this could hang.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>