Potential fatal object type mismatch in wp_set_object_terms()

[tellyworth]

The wp_set_object_terms() function calls wp_insert_term() and then immediately assumes the return result is an array, around line 1121:

		if ( !$id = is_term($term, $taxonomy) )
			$id = wp_insert_term($term, $taxonomy);
		$term_ids[] = $id['term_id'];

But wp_insert_term() doesn't always return an array, it can return a WP_Error object. This causes a fatal error:

PHP Fatal error: Cannot use object of type WP_Error as array in wordpress/wp-includes/taxonomy.php on line 1122

This turned up while I was writing unit tests. I'm not quite sure what the cause is yet because I can't backtrace a fatal error. It might not happen in normal operation, but I don't think a blind assumption like that is safe considering the consequences.

[tellyworth]

Found the trigger - my fault, a filter gone bad that was causing a NULL category name. Nevertheless, I think the code still needs an error check.

[tellyworth]

The above patch fixes it. I reproduced the same trigger and confirmed that there's no fatal error with the patch applied.

[ryan]

(In [6271]) Catch WP_Error. Props tellyworth. fixes #5226