Added corrected main-race.c

hw8/simu1/main-race.c

 #include "mythreads.h"
 
 int balance = 0;
+pthread_mutex_t mut;
 
 void* worker(void* arg) {
+    pthread_mutex_lock(&mut);
     balance++; // unprotected access
+    pthread_mutex_unlock(&mut);
     return NULL;
 }
 
 int main(int argc, char *argv[]) {
     pthread_t p;
     Pthread_create(&p, NULL, worker, NULL);
+    pthread_mutex_lock(&mut);
     balance++; // unprotected access
+    pthread_mutex_unlock(&mut);
     Pthread_join(p, NULL);
     return 0;
 }