security/libssh2: Backport upstream commits

Backport upstream commit 5d03b4f94ac6740e0085a6acda6fc417ca6ecc83 and
d4e43c7b69d10b6cfaaa53787addfd4beac7a3ce

References:
https://github.com/libssh2/libssh2/commit/5d03b4f94ac6740e0085a6acda6fc417ca6ecc83
https://github.com/libssh2/libssh2/commit/d4e43c7b69d10b6cfaaa53787addfd4beac7a3ce

Approved by: portmgr (blanket, build fix)