fix(lab02): avoid to free a null pointer

src/01-skeleton/s02e04-dynamic_allocation.c

@@ -49,8 +49,10 @@ void dynAlloc_exit(void) {
         e = list_entry(list_unique_elements.next, struct element, node);
         pr_info ("delete element %d: %s\n", e->unique_number, e->text);
         list_del(&e->node);
+        if (e != 0) {
             kfree(e);
         }
+    }
 
     pr_info("Memory allocated for dynamic allocation and linked list freed\n");
 }