Prevent fatal errors caused by admin-ajax.php's use of wp_insert_term()

[mdawaffe]

wp_insert_term() can return a WP_Error, but admin-ajax.php does not catch these errors. In two places (when adding post and link categories during post/link creation), the result of wp_insert_term() is blindly assumed to be an array, which will cause a fatal error.

Attached checks for errors and skips that category. The patch does not implement any error messaging to the user.

See also #17936.

[mdawaffe]

#17939 is related, and has a comprehensive patch for both of the issues.

[ryan]

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