CLSA-2026-1779467653 libssh: Fix of 4 CVEs

CVE-2025-4877: prevent base64 integer overflow and potential OOB write - CVE-2025-4878: initialize stack pointers to mitigate use of uninitialized values in legacy privatekeyfromfile path - CVE-2025-8277: fix DH-GEX packet filter and free unused ephemeral / ECDH keys to prevent memory exhaustion...