Bring the memory output inline with the suggestions in #3465.

Excess bytes, when one block is longer than the other, are not explicitly
highlighted.

The NULL / zero length block output has been cleaned up.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3515)