Small tweak to idle_loop()

In case of a succesful late join we set again
'searching' flag, so we can restart search
immediately without an useless lock/unlock
cycle.

No functional change.

src/search.cpp

@@ -1425,7 +1425,7 @@ void Thread::idle_loop() {
   while (!exit)
   {
       // If this thread has been assigned work, launch a search
-      if (searching)
+      while (searching)
       {
           Threads.mutex.lock();