Fix a crash in EVP_CIPHER_CTX_cleanup due to cipher_data may be NULL

or EVP_CTRL_INIT/EVP_CTRL_COPY was not called or failed.
If that happens in EVP_CipherInit_ex/EVP_CIPHER_CTX_copy set cipher = NULL,
aes_gcm_cleanup should check that gctx != NULL before calling OPENSSL_cleanse.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2562)