daemon: Fix crash when typing password at screen lock

open_reauthentication_requests is a hashtable which contains
GDBusMethodInvocations as values. We insert them in the hashtable with
only one reference, the one coming from the D-Bus method call.

But when a reauthentication starts,
gdm_dbus_manager_complete_open_reauthentication_channel() is called
which internally calls g_dbus_method_invocation_return_value().
This will eat the only reference, and we'll then try to remove the
invocation from the hash table, unref'ing a dead object.

Instead, remove the invocation from the hashtable without unref'ing
(once too much).

https://bugzilla.gnome.org/show_bug.cgi?id=733485