BUGFIX: setting currentIndex when going Up causes wrong scope.

When using Up in a loop opened with Up.Var is setting currentIndex wrong.
Without it, scopes work as expected.

Test case

```
example_controller.php:
...
public function loopA() {
	return new ArrayList(array(
		new ArrayData(array(
				'myVal' 	=> 'A.A',
		)),
		new ArrayData(array(
				'myVal' 	=> 'A.B',
		))
	));		
}
public function loopB() {
	return new ArrayList(array(
		new ArrayData(array(
				'myVal' 	=> 'B.A',
		)),
		new ArrayData(array(
				'myVal' 	=> 'B.B',
		))
	));
}
public function myVal() {
	return 'zero';
}
...
```

```
example_template.ss
...
<pre>
<% loop loopA %>
myVal: $myVal
	<% loop Up.loopB %>
	myVal: $myVal  Up myVal: $Up.myVal
	<% end_loop %>
<% end_loop %>
</pre>
...
```

```
Expected output of template:
myVal: A.A
	myVal: B.A  Up myVal: A.A
	myVal: B.B  Up myVal: A.A

myVal: A.B
	myVal: B.A  Up myVal: A.B
	myVal: B.B  Up myVal: A.B


Actual output of template:
myVal: A.A
	myVal: B.A  Up myVal: zero
	myVal: B.B  Up myVal: zero

myVal: A.B
	myVal: B.A  Up myVal: zero
	myVal: B.B  Up myVal: zero
```

Fix is quick and not tested enough... but it solves the above problem.
SSViewer_Scope stack logic and stack indexes are pretty complex and poorly documented in the code, which makes this fix a bit problematic...
Anyway, at least bug is obvious and repeatable.