stack-tracker: Fix an off-by-one error in restack_managed

When restacking the last window alone, we would trigger this off-by-one
error. This would throw us off the end of the array, causing lower_below
warnings for nonsensical values.

Since the last window already is lowered below everything else, we
shouldn't need to lower it.

src/core/stack-tracker.c

@@ -1107,7 +1107,7 @@ meta_stack_tracker_restack_managed (MetaStackTracker *tracker,
       new_pos--;
     }
 
-  while (new_pos >= 0)
+  while (new_pos > 0)
     {
       meta_stack_tracker_lower_below (tracker, managed[new_pos], managed[new_pos - 1]);
       new_pos--;