Update listdir.c

1.close dir before return;
2.fixed indentation.

examples/file/listdir.c

@@ -25,6 +25,7 @@ void list_dir(const char* path)
         fullpath = rt_malloc(256);
         if (fullpath == RT_NULL)
         {
+            closedir(dir);
             rt_kprintf("no memory\n");
             return;
         }