FreeBSD -- OpenSSH pre-authentication double free

Problem Description:
A flaw in the backwards-compatibility key exchange route allows a
pointer to be freed twice.
Impact:
A remote, unauthenticated attacker may be able to cause a denial of
service, or possibly remote code execution.
Note that FreeBSD 12.3 and FreeBSD 13.1 include older versions of
OpenSSH, and are not affected. FreeBSD 13.2-BETA1 and later include the
fix.