Skip XRef reconstruction for new-style XRef streams
XRef::constructXRef was invented to support old style XRef tables. Sadly it won't work for new style XRef streams. If applied anyway, it corrupts our existing
XRef::entries array. Better skip reconstruction for the XRef-stream-case in
XRef::readXRefUntil (just like
XRef::fetch already does).
Long term goal should be to implement a
XRef::constructXRef variant that can cope with XRef streams.