Fix some undefined behaviour in the Curve448 code (2nd attempt)

Fixes #6800
Replaces #5418

This commit reverts commit 7876dbff and moves the check for a
zero-length input down the callstack into sha3_update().

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/6838)