Avoid invoking memcpy if size is zero or the supplied buffer is NULL

Avoid invoking memcpy if size is zero or the supplied buffer is NULL

This allows for passing a NULL pointer with zero max_len.

Invoking memcpy on NULL is undefined behaviour, even if the size is zero.

https://en.cppreference.com/w/c/string/byte/memcpy

The function can now be queried for the necessary buffer length.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10541)