message: Fixed length error after append new string.
diff: @@ -151,8 +151,9 @@ static PHP_METHOD(swoole_buffer, append)
  {
  zend_update_property_long(swoole_buffer_class_entry_ptr, getThis(), ZEND_STRL("capacity"), buffer->size TSRMLS_CC);
  }
  - zend_update_property_long(swoole_buffer_class_entry_ptr, getThis(), ZEND_STRL("length"), buffer->length TSRMLS_CC);
  - RETURN_LONG(buffer->length);
  + zend_update_property_long(swoole_buffer_class_entry_ptr, getThis(), ZEND_STRL("length"),
  + buffer->length - buffer->offset TSRMLS_CC);
  + RETURN_LONG(buffer->length - buffer->offset);
  }
  else
  {
  