Rename EVP_CIPHER_CTX_cipher_data to EVP_CIPHER_CTX_get_cipher_data

We had the function EVP_CIPHER_CTX_cipher_data which is newly added for
1.1.0. As we now also need an EVP_CIPHER_CTX_set_cipher_data it makes
more sense for the former to be called EVP_CIPHER_CTX_get_cipher_data.

Reviewed-by: Tim Hudson <tjh@openssl.org>