Minor fix in loop range that guarantees no Segfaults could happen.

include/ThreadPool.h

@@ -67,7 +67,7 @@ void ThreadPool::thread_worker() {
 }
 
 void ThreadPool::join_all() {
-  for (long i = 0; i < numOfThreads; i++) {
+  for (long i = 0; i < static_cast<long>(_threads.size()); i++) {
     if (_threads[i].joinable())
       _threads[i].join();
   }