Add a limit to the number of results in _node_reference_options().

When _node_reference_options() does execute node_reference_potential_references(), it doesn't pass in any options, wich means that the defaults are used.

The default set limit to 0, which means no limitation at all.

If you have ~400k nodes, like we do, then it explodes.

The attached patch sets the default limit to 10 instead, not sure if a different value should be chosen, but there needs to be something.