mutex(fix): fixed race condition (part 2)

In addition to the previous commit, this commit makes threads wait for
the thread manager to say it took the mutex from them before they can
ask for it again.

main.cpp

@@ -56,9 +56,10 @@ void *thread_task_increment(const struct thread_data &thread) {
     {
 
       // tell the thread manager we want the mutex
+      while (!*thread.manager_took_mutex)
       (*thread.wants_mutex) = true;
       // block until we have the mutex
-      while (!*thread.has_mutex && *thread.manager_took_mutex)
+      while (!*thread.has_mutex)
         ;
 
       // enter the mutex