fixed local DoS when UnregisterHandler was called for a not existing handler

Any user with DBUS access could cause a SEGFAULT in tcmu-runner by
running something like this:

dbus-send --system --print-reply --dest=org.kernel.TCMUService1 /org/kernel/TCMUService1/HandlerManager1 org.kernel.TCMUService1.HandlerManager1.UnregisterHandler string:123
on_handler_vanished(GDBusConnection *connection,
		    const gchar     *name,
		    gpointer         user_data)
{
	struct tcmur_handler *handler = user_data;
	struct dbus_info *info = handler->opaque;

	if (info->register_invocation) {
		char *reason;
		reason = g_strdup_printf("Cannot find handler bus name: "
				"org.kernel.TCMUService1.HandlerManager1.%s",
				handler->subtype);
		g_dbus_method_invocation_return_value(info->register_invocation,
			    g_variant_new("(bs)", FALSE, reason));
		g_free(reason);
	}
	tcmur_unregister_handler(handler);
	dbus_unexport_handler(handler);
}

on_handler_vanished(GDBusConnection *connection,
		    const gchar     *name,
		    gpointer         user_data)
{
	struct tcmur_handler *handler = user_data;
	struct dbus_info *info = handler->opaque;

	if (info->register_invocation) {
		char *reason;
		reason = g_strdup_printf("Cannot find handler bus name: "
				"org.kernel.TCMUService1.HandlerManager1.%s",
				handler->subtype);
		g_dbus_method_invocation_return_value(info->register_invocation,
			    g_variant_new("(bs)", FALSE, reason));
		g_free(reason);
	}
	tcmur_unregister_handler(handler);
	dbus_unexport_handler(handler);
}
