This demo optionally accepts a single command line argument, allowing
the output length to be specified.

Fixes #14106.
Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NPaul Dale <pauli@openssl.org>
Reviewed-by: NTomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17803)