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).
Fixes #139 (closed) and Okular bug 395660. There had been multiple patch variants. Follow discussion in #139 (closed) to see why the approach presented here is probably the best.
Long term goal should be to implement a XRef::constructXRef variant that can cope with XRef streams.