ssh: Gracefully handle failing g_hmac_new()
ssh: Gracefully handle failing g_hmac_new()

This function could fail if glib doesn't support HMAC or SHA1, e. g.
because it is in FIPS mode.

This only affects building against libssh 0.7, i. e. RHEL/CentOS 7 and
Debian 9. In newer operating systems with libssh 0.8, this entire file
is not built.

Fixes #12016
https://gitlab.gnome.org/GNOME/glib/merge_requests/897

cockpit-project/cockpit,deryni/cockpit,cockpituous/cockpit,deryni/cockpit,cockpit-project/cockpit,deryni/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,garrett/cockpit,garrett/cockpit,cockpituous/cockpit,martinpitt/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,garrett/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpituous/cockpit,deryni/cockpit,deryni/cockpit,mvollmer/cockpit,cockpituous/cockpit,deryni/cockpit,cockpit-project/cockpit,garrett/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,mvollmer/cockpit,garrett/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,deryni/cockpit