Make sure the $parent argument of wp_insert_attachment() still works as expected after [28579].

Merges [29745] to the 4.0 branch.

props jesin, dikiy_forester.
fixes #29646.

Built from https://develop.svn.wordpress.org/branches/4.0@30258


git-svn-id: http://core.svn.wordpress.org/branches/4.0@30258 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/post.php

@@ -4719,10 +4719,15 @@ function is_local_attachment($url) {
 function wp_insert_attachment( $args, $file = false, $parent = 0 ) {
 	$defaults = array(
 		'file'        => $file,
-		'post_parent' => $parent
+		'post_parent' => 0
 	);
+
 	$data = wp_parse_args( $args, $defaults );
 
+	if ( ! empty( $parent ) ) {
+		$data['post_parent'] = $parent;
+	}
+
 	$data['post_type'] = 'attachment';
 
 	return wp_insert_post( $data );