Fix a gcc 4.7 warning

New gcc 4.7 complains about casting a volatile pointer
to void* so assign the variables directly.

No functional change.

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

src/thread.cpp

@@ -446,7 +446,8 @@ void ThreadsManager::start_thinking(const Position& pos, const LimitsType& limit
   SearchMoves = searchMoves;
 
   // Reset signals before to start the new search
-  memset((void*)&Signals, 0, sizeof(Signals));
+  Signals.stopOnPonderhit = Signals.firstRootMove = false;
+  Signals.stop = Signals.failedLowAtRoot = false;
 
   main.do_sleep = false;
   cond_signal(&main.sleepCond); // Wake up main thread and start searching