FreeBSD: VID-57b1ee25-1a7c-11ec-9376-0800272221cc (CVE-2021-3634): libssh -- possible heap-buffer overflow vulnerability

libssh security advisories: The SSH protocol keeps track of two shared secrets during the lifetime of the session. One of them is called `secret_hash` and and the other `session_id`. Initially, both of them are the same, but after key re-exchange, previous `session_id` is kept and used as an input to new `secret_hash`. Historically, both of these buffers had shared length variable, which worked as long as these buffers were same. But the key re-exchange operation can also change the key exchange method, which can be based on hash of different size, eventually creating `secret_hash` of different size than the `session_id` has. This becomes an issue when the `session_id` memory is zeroized or when it is used again during second key re-exchange.