Better fix of crash bug

Crash is due to slave thread accessing main thread
data while exiting.

Fix it in the proper way makeing slave threads
independent from main state.

No functional change.

src/thread.cpp

@@ -123,7 +123,7 @@ void Thread::idle_loop() {
   {
       std::unique_lock<Mutex> lk(mutex);
 
-      while (!Threads.main()->thinking && !exit)
+      while (!searching && !exit)
           sleepCondition.wait(lk);
 
       lk.unlock();
@@ -190,10 +190,8 @@ void ThreadPool::exit() {
   timer = nullptr;
 
   for (Thread* th : *this)
-      if (th != Threads.main())
       delete_thread(th);
 
-  delete_thread(Threads.main()); // Must be the last one
   clear(); // Get rid of stale pointers
 }