invalid node_load in nat_get_nids

Subtle bug : happens when nat_get_nids's static cache has been filled with $get_nodes = FALSE, and is being used by a a later call with $get_nodes = TRUE.
$nid_cache stores titles as values, nids are the keys.
Currently, the function calls node_load($title), which of course fails with a bang (invalid mySQL request).

Attached patch fixes this.