modes/ocb128.c: fix misaligned access in ILP32 builds on 64-bit processors.

One could have fixed the problem by arranging 64-bit alignment of
EVP_AES_OCB_CTX.aad_buf in evp/e_aes.c, but CRYPTO_ocb128_aad
prototype doesn't imply alignment and we have to honour it.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2994)