ssl/s3_enc.c: fix logical errors in ssl3_final_finish_mac.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7085)

(cherry picked from commit 7d0effeacbb50b12bfc24df7614d7cf5c8686f51)