Fix exception when parsing empty TAG_Compounds

Before:

    Equipment: [5 TAG_Compound(s)]
        : {0 Entries}
Traceback (most recent call last):
  File "./norbert.py", line 595, in <module>
    sys.exit(main())
  File "./norbert.py", line 155, in main
    r = norbert(nbtfile, options, arg)
  File "./norbert.py", line 341, in norbert
    print_subtags(tag, maxdepth=options.maxdepth, format=options.format)
  File "./norbert.py", line 507, in print_subtags
    pre_action=print_tag_pre, post_action=print_tag_post)
  File "./norbert.py", line 473, in traverse_subtags
    push_child(stack, cur, 0)
  File "./norbert.py", line 500, in push_child
    stack.append( (parent.tags[i], None) )

After:

    Equipment: [5 TAG_Compound(s)]
        : {0 Entries}
        : {0 Entries}
        : {0 Entries}
        : {0 Entries}
        : {0 Entries}