[bugfix canary] failing test showing infinite recursion w/_super

restores a bit of code removed in #9651 that would set the `__nextSuper` to null. It's not there on master currently, causing an infinite loop to occur.

cc @stefanpenner @mixonic 

refs emberjs/ember.js PR #9651
I feel guilty even asking, but can this go inside the `if (func) {`? It would avoid checking `length` if there is no `func`.