In get_category(), check for empty category id so that we do not create a malformed SQL query.

git-svn-id: https://develop.svn.wordpress.org/trunk@2509 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/functions.php

@@ -617,6 +617,9 @@ function &get_page(&$page, $output = OBJECT) {
 function &get_category(&$category, $output = OBJECT) {
 	global $cache_categories, $wpdb;
 
+	if ( empty($category) )
+		return null;
+
 	if ( ! isset($cache_categories))
 		update_category_cache();