ec/ecp_nistz256.c: fix ecp_nistz256_set_from_affine.

ecp_nistz256_set_from_affine is called when application attempts to use
custom generator, i.e. rarely. Even though it was wrong, it didn't
affect point operations, they were just not as fast as expected.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6738)