Fix dynamically calcluated list height bug when in T2B mode and top val given

In T2B (top to bottom centering) mode, the 'top' parametr value isn't actually
for the top of the window position (later), so just zero it here in case it
was specified (and non-zero) to correct the calcutation on the next lines:
```
	if(top+height>s_bottom)
		height=(s_bottom)-top;
```