Rename SplitPoint parentSstack

Now that we don't have anymore a search stack array in
SplitPoint we can rename this data member to somthing more
usual.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/thread.h

@@ -60,7 +60,7 @@ struct SplitPoint {
 
   // Const pointers to shared data
   MovePicker* mp;
-  SearchStack* parentSstack;
+  SearchStack* ss;
 
   // Shared data
   Lock lock;