Hang when quickly moving between text nodes

[jwilk]

Aleš Kapica reported that if you use keyboard to move between text nodes, djvusmooth can fall into an infinite loop.

To reproduce:

• Open a document with lots of hidden text (such as djvu3spec.djvu).
• Go to the "Text" tab.
• Open the "page" node.
• Start pressing ↓ vigorously.