If we ever forget to fill it in the randoms, they'll end up all zero.
Particularly at the ClientHello, that logic is getting increasingly far
away from ClientHello serialization, so add a test to make sure we
notice.
(This will flakily fail with probability 2^-256, which is reasonably
unlikely.)
Change-Id: I81f32fd96dbccf377cb92198a222b557ab66976b
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/47665
Reviewed-by: Adam Langley <agl@google.com>
David Benjamin