RT3841: memset() cipher_data when allocated

If an EVP implementation (such as an engine) fails out early, it's
possible to call EVP_CIPHER_CTX_cleanup() which will call
ctx->cipher->cleanup() before the cipher_data has been initialized
via ctx->cipher->init().  Guarantee it's all-bytes-zero as soon as
it is allocated.

Reviewed-by: Matt Caswell <matt@openssl.org>