Observe.List sort plugin erroring on item removal

Ran into this when testing some new functionality when implementing the sort plugin for Observe.List (in conjunction on a Model.List).

When removing the last item in the list, an error is thrown stating that the "item is undefined".

In "can/observe/sort/sort.js", lines 142-170:
```javascript
proto._changes = function(ev, attr, how, newVal, oldVal){
	if(this.comparator && /^\d+./.test(attr) ) {
		// get the index
		var index = +/^\d+/.exec(attr)[0],
			// and item
			item = this[index],
			// and the new item
			newIndex = this.sortedIndex(item);

		if(newIndex !== index){
			// move ...
			[].splice.call(this, index, 1);
			[].splice.call(this, newIndex, 0, item);

			can.trigger(this, "move", [item, newIndex, index]);
			ev.stopImmediatePropagation();
			can.trigger(this,"change", [
				attr.replace(/^\d+/,newIndex),
				how,
				newVal,
				oldVal
			]);
			
			return;
		}
	}
	
	_changes.apply(this, arguments);
};
```
At line 149, `newIndex = this.sortedIndex(item)` throws an error because `item` doesn't exist in the List any longer. A little check to see if `typeof item != "undefined"` fixes the issue.

Revised method:
```javascript
proto._changes = function(ev, attr, how, newVal, oldVal){
	if(this.comparator && /^\d+./.test(attr) ) {
		// get the index
		var index = +/^\d+/.exec(attr)[0],
			// and item
			item = this[index];

		if( typeof item != 'undefined') {
		  // and the new item
		  var newIndex = this.sortedIndex(item);
		  
		  if(newIndex !== index){
		    // move ...
		    [].splice.call(this, index, 1);
		    [].splice.call(this, newIndex, 0, item);
		    
		    can.trigger(this, "move", [item, newIndex, index]);
		    ev.stopImmediatePropagation();
		    can.trigger(this,"change", [
		                                attr.replace(/^\d+/,newIndex),
		                                how,
		                                newVal,
		                                oldVal
		                                ]);
		    
		    return;
		  }
		}
	}
	
	_changes.apply(this, arguments);
};
```