rsa/rsa_oaep.c: remove memcpy calls from RSA_padding_check_PKCS1_OAEP.

And make RSAErr call unconditional.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit 75f5e944)

(Merged from https://github.com/openssl/openssl/pull/7735)