Fix incorrect StateInfo

We use Position::set() to set root position across
threads. But there are some StateInfo fields (previous,
pliesFromNull, capturedPiece) that cannot be deduced
from a fen string, so set() clears them and to not lose
the info we need to backup and later restore setupStates->back().
Note that setupStates is shared by threads but is accessed
in read-only mode.

This fixes regression introduced by df6cb446ea

Tested with 3 threads at STC:
LLR: 2.95 (-2.94,2.94) [-4.00,0.00]
Total: 14436 W: 2304 L: 2196 D: 9936

Bench: 5608839

src/thread.cpp

@@ -158,10 +158,12 @@ void ThreadPool::start_thinking(Position& pos, StateListPtr& states,
   if (states.get())
       setupStates = std::move(states); // Ownership transfer, states is now empty
 
-  // We use Position::set() to set root position across threads. So we
+  // We use Position::set() to set root position across threads. But there are
-  // need to save and later to restore st->previous, cleared by set().
+  // some StateInfo fields (previous, pliesFromNull, capturedPiece) that cannot
-  // Note that setupStates is shared by threads but is accessed in read-only mode.
+  // be deduced from a fen string, so set() clears them and to not lose the info
-  StateInfo* previous = setupStates->back().previous;
+  // we need to backup and later restore setupStates->back(). Note that setupStates
+  // is shared by threads but is accessed in read-only mode.
+  StateInfo tmp = setupStates->back();
 
   for (Thread* th : Threads)
   {
@@ -171,7 +173,7 @@ void ThreadPool::start_thinking(Position& pos, StateListPtr& states,
       th->rootPos.set(pos.fen(), pos.is_chess960(), &setupStates->back(), th);
   }
 
-  setupStates->back().previous = previous;
+  setupStates->back() = tmp;
 
   main()->start_searching();
 }