Fix annoying error in log

lib/migrate.php

@@ -91,7 +91,8 @@ class OC_Migrate{
 	 	if( self::$exporttype == 'user' ){
 	 		// Check user exists
 	 		if( !is_null($uid) ){
-		 		if( !OC_User_Database::userExists( $uid ) ){
+                                $db = new OC_User_Database;
+		 		if( !$db->userExists( $uid ) ){
 					OC_Log::write('migration', 'User: '.$uid.' is not in the database and so cannot be exported.', OC_Log::ERROR);
 					return json_encode( array( 'success' => false ) );
 				}