Stop using CRYPTO_gcm128_init() and stack allocated GCM128_CONTEXT.
Since struct gcm128_context is not exposed via a public header, there is no
way CRYPTO_gcm128_init() can actually be used properly. Instead, use
CRYPTO_gcm128_new() and CRYPTO_gcm128_free_bird()^WCRYPTO_gcm128_release()
(naming consistency is apparently hard).