update output function and fix segfault

list_movers.c

@@ -6,14 +6,20 @@ int ListCount(LIST *list) {
 }
 
 void *ListFirst(LIST *list) {
+  if (list->size != 0) {
     list->current = list->head;
     return list->current->data;
   }
+  return NULL;
+} 
 
 void *ListLast(LIST *list) {
+  if (list->size != 0) {
     list->current = list->tail;
     return list->current->data;
   }
+  return NULL;
+}
 
 void *ListNext(LIST *list) {
   if (list->current->next != NULL) {

main.c

@@ -69,12 +69,16 @@ void *output(void *arg) {
   char msg[30];
   while (1) {
     pthread_mutex_lock(&receivemut);
+    if (ListCount(receivelist) != 0) {
       ListFirst(receivelist);
       strcpy(msg, ListCurr(receivelist));
       ListRemove(receivelist);
+    }
     pthread_mutex_unlock(&receivemut);
+    if (ListCount(receivelist) != 0) {
       printf("%s\n", msg);
     }
+  }
 
   return 0;
 }