Signaling abort before nulling out onging event in #abort-a-navigateevent can trigger assert in firing navigate event

### What is the issue with the HTML Standard?

[This](https://html.spec.whatwg.org/#navigate-event-firing:assert-7) assert in particular.

The solution is to move step 4 before step 2 in https://html.spec.whatwg.org/#abort-a-navigateevent.