Fix notice that exists for plugins with no slug

Not all plugins have a slug, especially plugins not from the WordPress.org repository. 

Props imath for initial patch. 
Fixes #31694


Built from https://develop.svn.wordpress.org/trunk@31834


git-svn-id: http://core.svn.wordpress.org/trunk@31816 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/class-wp-plugins-list-table.php

@@ -515,10 +515,11 @@ class WP_Plugins_List_Table extends WP_List_Table {
 		if ( ! empty( $totals['upgrade'] ) && ! empty( $plugin_data['update'] ) )
 			$class .= ' update';
 
+		$plugin_slug = ( isset( $plugin_data['slug'] ) ) ? $plugin_data['slug'] : '';
 		printf( "<tr id='%s' class='%s' data-slug='%s'>",
 			$id,
 			$class,
-			$plugin_data['slug']
+			$plugin_slug
 		);
 
 		list( $columns, $hidden ) = $this->get_column_info();

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.2-beta1-31833';
+$wp_version = '4.2-beta1-31834';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.