Move some stuff out of lock protection in split()

We shouldn't need lock protection to increment
splitPointsCnt and set curSplitPoint of masterThread.

Anyhow because this code is very tricky and prone to
races bound the change in a single patch.

No functional change.

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

src/search.cpp

@@ -1906,9 +1906,6 @@ void Thread::idle_loop(SplitPoint* sp_master) {
               Threads[master].wake_up();
       }
   }
-  // In helpful master concept a master can help only a sub-tree of its split
-  // point, and because here is all finished is not possible master is booked.
-  assert(!is_searching);
 }