Zhonya's, though is an AP item, it is not at all bad if you got more than enough gold. The active is pretty effective if you know how to time it.
But let's get a little more realistic and practical, by buying your own
.
Some ADCs can also just run away when Rengar shows up, like any flashes/dashes, Vayne's condemn, and the like. Otherwise you can't be on your own.
And lastly, don't go out alone. You know what they say about singled-out targets, those are assassins' favorites. Stay close just enough so someone else can save you, because it is usually a given that Rengars go after the carries, and not supports/tanks. Of course they're not 100% reliable when it comes to saving you (because sometimes I just am too slow to "see" Rengar coming), but still better than nothing.
No really, no matter how much you wanted to splitpush, never go alone.