Add EVP_AEAD_CTX_zero.

Match the other stack-allocated types in that we expose a wrapper function to
get them into the zero state. Makes it more amenable to templates like
ScopedOpenSSLContext.

Change-Id: Ibc7b2b1bc0421ce5ccc84760c78c0b143441ab0f
Reviewed-on: https://boringssl-review.googlesource.com/5753
Reviewed-by: Adam Langley <agl@google.com>
4 files changed