Allow Bitbases::init() to be called more than once

Currently if we call it more than once, we crash.

This is not a real problem, because this function is
indeed called just once. Nevertheless with this small fix,
that gets rid of a hidden 'static' variable, we cleanly
resolve the issue.

While there, fix also ThreadPool::exit to return in a
consistent state. Now all the init() functions but
UCI::init() are reentrant and can be called multiple
times.

No functional change.

src/thread.cpp

@@ -299,9 +299,12 @@ void ThreadPool::init() {
 void ThreadPool::exit() {
 
   delete_thread(timer); // As first because check_time() accesses threads data
+  timer = nullptr;
 
   for (Thread* th : *this)
       delete_thread(th);
+
+  clear(); // Get rid of stale pointers
 }