Filter iterates through invalid field values

It's possible to have field values like [add_more] => Add another item. These need to be skipped so that we're looking at only numeric keys in each field's array for each language.

Currently, this produces the following on an attempt to save a node:

PHP Fatal error: Cannot create references to/from string offsets nor overloaded objects in /path/to/drupal/sites/all/modules/htmltidy/htmltidy.node.inc on line 87, referer: http://www.example.com/node/99999/edit