add benchmark for serialize addon

addons/xterm-addon-serialize/src/SerializeAddon.ts

@@ -182,7 +182,7 @@ export class SerializeAddon implements ITerminalAddon {
       throw new Error('Cannot use addon until it has been loaded');
     }
 
-    const maxRows = this._terminal.rows;
+    const maxRows = this._terminal.buffer.length;
     const handler = new StringSerializeHandler(this._terminal.buffer);
 
     rows = crop(rows, 0, maxRows, maxRows);