typo in qqueue.c file

[Charles]

line81-86 can be modified to:

 *  obj = queue->pop(queue, NULL);
 *  printf("pop(): %s\n", (char*)obj);
 *  free(obj);
 *  obj = queue->pop(queue, NULL);
 *  printf("pop(): %s\n", (char*)obj);
 *  free(obj);

[Seungyoung Kim (Steve)]

Thank you.