Observe the following multi-thread safe list insertion code: typedef struct _node_t { int key; struct __node_t *next; } node_t; mutex_t m = PTHREAD_MUTEX_INITIALIZER; node t *head = NULL; int List Insert (int key) { mutex lock (&m); node_t *n malloc(sizeof (node_t)); if (n= NULL) { return -1; } //fail to insert n->key = key; } n->next = head; head = n; mutex_unlock (&m); return 0; // insert // success The code has a big bug. Where is that big bug (only one)? Fix the code and make it work correctly.