ssh-agent: Fix crash by uninitialized GMutex

This change fixes a crash (caused by NULL-pointer dereference) on systems where GLib uses POSIX Threads based GMutex implementation.

OpenIndiana: Bug #10209: Process gnome-keyring-daemon terminated by SIGSEGV on MATE start

FreeBSD: Bug 253359 - security/gnome-keyring: gnome-keyring-daemon segmentation fault when the mate session closes

Fixes #65 (closed).