Fix incorrect test for overflow in ihu_interval.
Fix incorrect test for overflow in ihu_interval.

woniullb/babeld,wlanslovenija/babeld,boutier/babeld,jech/babeld,tcatm/babeld,Drooids/babeld,jech/babeld,dtaht/babeld-shortrtt-metrics,Gwendocg/babeldToS,sudomesh/babeld,wlanslovenija/babeld,Gwendocg/babeldToS