stop morph.html being called from a bound view in a destroying state

I've been experiencing this bug where update is being called of a bound view in a destroying state:

Here is update:
```
update: function () {
  this.updateId = null;
  this.morph.html(this.render());
},
```
If update is called on a view in this state, ```this.moprh``` is null so a call to html will obviously blow up.

My naive fix just checks for this.morph to be truthy:
```
update: function () {
  this.updateId = null;
  // FIXME: Update should not be called on a view with a state of destroying
  if(!this.morph){
    return;
  }
  this.morph.html(this.render());
},
```

I think something more sinister is wrong in that update should not be called in this scenario.  

I created an issue for this a couple of days ago #2815.

Maybe somebody can comment on this, it is happening with more than one bound helper I have.