Check that `$_POST` array index is set before comparing it in `post_preview()`.

This prevents PHP notices in cases where a user with 'edit_others_posts' but
without 'publish_posts' previews another user's posts.

Props tyxla.
Fixes #31760.
Built from https://develop.svn.wordpress.org/trunk@31896


git-svn-id: http://core.svn.wordpress.org/trunk@31875 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/post.php

@@ -1667,7 +1667,7 @@ function post_preview() {
 	} else {
 		$is_autosave = true;
 
-		if ( 'auto-draft' == $_POST['post_status'] )
+		if ( isset( $_POST['post_status'] ) && 'auto-draft' == $_POST['post_status'] )
 			$_POST['post_status'] = 'draft';
 
 		$saved_post_id = wp_create_post_autosave( $post->ID );

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.2-beta2-31895';
+$wp_version = '4.2-beta2-31896';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.