tty: Fix slave peer index accounting

We need to test every even index (which represents a slave
peer) thus use modulo operation here.

Reported-by: Alexander Kartashov <alekskartashov@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>