Don't free in cleanup routine

Cleanse instead, and free in the free routine.

Seems to have been introduced in commit
846ec07d

 when EVP_CIPHER_CTX was made
opaque.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2798)