update output function and fix segfault

list_movers.c

@@ -6,13 +6,19 @@ int ListCount(LIST *list) {
 }
 
 void *ListFirst(LIST *list) {
-  list->current = list->head;
+  if (list->size != 0) {
-  return list->current->data;
+    list->current = list->head;
-}
+    return list->current->data;
+  }
+  return NULL;
+} 
 
 void *ListLast(LIST *list) {
-  list->current = list->tail;
+  if (list->size != 0) {
-  return list->current->data;
+    list->current = list->tail;
+    return list->current->data;
+  }
+  return NULL;
 }
 
 void *ListNext(LIST *list) {

main.c

@@ -69,11 +69,15 @@ void *output(void *arg) {
   char msg[30];
   while (1) {
     pthread_mutex_lock(&receivemut);
-    ListFirst(receivelist);
+    if (ListCount(receivelist) != 0) {
-    strcpy(msg, ListCurr(receivelist));
+      ListFirst(receivelist);
-    ListRemove(receivelist);
+      strcpy(msg, ListCurr(receivelist));
+      ListRemove(receivelist);
+    }
     pthread_mutex_unlock(&receivemut);
-    printf("%s\n", msg);
+    if (ListCount(receivelist) != 0) {
+      printf("%s\n", msg);
+    }
   }
 
   return 0;