security/libssh2: Fix CVE-2026-7598

Backport upstream commit 256d04b60d80bf1190e96b0ad1e91b2174d744b1
to fix CVE-2026-7598

Reference:
https://github.com/libssh2/libssh2/commit/256d04b60d80bf1190e96b0ad1e91b2174d744b1

PR: 295742
Approved by: Maintainer timeout, 2+ weeks