Document EC_POINT_get_affine_coordinates_*.

In particular, x and y may be NULL, as used in ecdsa_ossl.c. Make use of
this in ecdh_ossl.c as well, to save an otherwise unnecessary temporary.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5532)