check if interface is known when using use_vmac

vrrp->ifp is NULL when use_vmac keyword is defined before the
interface keyword. This would result in a segfault
(http://sourceforge.net/p/keepalived/mailman/message/33083027/).

I think it's better to show a message than segfaulting. Not sure if it's better to log to ERR instead of INFO. Not 100% sure if this solution is the best way to go. Let me know if this pull request needs to be changed.