Directly return from final sha3/keccak_final if no bytes are requested

Requesting zero bytes from shake previously led to out-of-bounds write
on some platforms.

Signed-off-by: Patrick Steuer <patrick.steuer@de.ibm.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9433)