Warwick's howl's DMG reduction should last for an additional 0.1 seconds if he pops it early.

So that if he pops it early, to use its CC to interrupt an enemy channeled ability, such as Nunu's ult, then the damage reduction will linger on him for an additional 0.1 seconds, just long enough to do its damage block on any damage that immediately occurs on the very next frame after he pops it.

But if he lets the howl go the full duration, then there won't be any extra 0.1 seconds of damage reduction, even if it triggers damage immediately after it ends.