modes/ctr128.c: pay attention to ecount_buf alignment in CRYPTO_ctr128_encrypt.

It's never problem if CRYPTO_ctr128_encrypt is called from EVP, because
buffer in question is always aligned within EVP_CIPHER_CTX structure.

RT#4218

Reviewed-by: Richard Levitte <levitte@openssl.org>
(cherry picked from commit 5e4bbeb4)