CVE-2021-3634: Medium severity libssh libssh vulnerability

A flaw has been found in libssh in versions prior to 0.9.6. The SSH protocol keeps track of two shared secrets during the lifetime of the session. One of them is called secrethash and the other sessionid. Initially, both of them are the same, but after key re-exchange, previous sessionid is kept and used as an input to new secrethash. 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 "secrethash" of different size than the sessionid has. This becomes an issue when the sessionid memory is zeroed or when it is used again during second key re-exchange.