fix for halved output data - set list head's prev to new tail

lab01/zad3/src/doubly_linked_list.c

@@ -38,6 +38,8 @@ void doubly_linked_list_insert(DoublyLinkedList *l, void *value) {
     tail->next = node_new(value, l->element_size);
     tail->next->next = l->head;
     tail->next->prev = tail;
+
+    l->head->prev = tail->next;
     l->length += 1;
 }