Add `@access private` to `_get_custom_object_labels()`, which was already implied by the dangling underscore. Cast `$labels` to `array` in `get_taxonomy_labels()` and `_get_custom_object_labels()`.

Props nacin, foofy.
Fixes #16310.



git-svn-id: https://develop.svn.wordpress.org/trunk@25553 602fd350-edb4-49c9-b593-d223f7449a82