Found by OSS-Fuzz. This comes up if you enable client certificates and
the draft ECH implementation on the server.
Bug: 275, oss-fuzz:35815
Change-Id: I0b4fcc994f7238f8a3cf1f1934672bac0cee0cfb
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/48425
Reviewed-by: Adam Langley <agl@google.com>