this change will prevent undefined behavior when src and dst are equal...

this change will prevent undefined behavior when src and dst are equal (memcpy), effectively allowing setting length only in both functions.

CLA: trivial

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2750)