Don't call memcpy with NULL as source
Don't call memcpy with NULL as source

Calling it with lenght 0 and NULL as source is undefined behaviour.

Reviewed-by: Rich Salz <c04971a99e5a9ee80eaab4b1deb37e845b0bd697@openssl.org>
GH: #2089

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl