Fixes compile error C3487 ('const Valgrind::XmlProtocol::Frame':
all return expressions in a lambda must have the same type: previously
it was 'Valgrind::XmlProtocol::Frame')

Change-Id: I6545cd2eda1fba4058bf0dc59ed835fed07f86e8
Reviewed-by: hjk <hjk@theqtcompany.com>