We fill in placeholder values of all zeros fairly often in TLS now,
as workarounds for messages being constructed in the wrong order.
draft-12 of ECH adds even more of these. Add a helper so we don't need
to interrupt an || chain with a memset.
Change-Id: Id4f9d988ee67598645a01637cc9515b475c1aec2
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/48909
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>
David Benjamin