Adding an existing category via the category meta box on the post edit screen fails

[mdawaffe]

To reproduce:

1. Create a post category called "test".
2. Go to post-new.php
3. In the category meta box, click "+ Add New Category"
4. Type "test" into the textbox and click "Add New Category"
5. Watch "Uncategorized" get added to the list. Look at the AJAX response and see that checkbox's category ID is "Array".

term_exists() can return an array or an ID. Attached checks for arrayness.

Caused by [12798]

Combine original patch with #17938

.2.diff looks good.

In [19792]:

Check the return of wp_insert_term() for WP_Error or array. Prevents fatal erros and failure to add categories when adding terms via ajax. Props mdawaffe. fixes #17938 #17939