PropagateDownload: fix possible crash

PropagateDownload: fix possible crash

Backtrace looks like this:
Crash: EXCEPTION_ACCESS_VIOLATION_READ at 0x0
  File "propagatedownload.cpp", line 234, in OCC::GETFileJob::slotReadyRead
  File "moc_propagatedownl_CA5CFSHZDTX34X.cpp", line 86, in OCC::GETFileJob::qt_static_metacall
  File "qobject.cpp", line 495, in QMetaCallEvent::placeMetaCall
  File "qobject.cpp", line 1256, in QObject::event
  File "qapplication.cpp", line 3804, in QApplicationPrivate::notify_helper

GETFileJob::slotReadyRead can be called with a QueuedConnection when the
bendwith manager is involved. In that case, if the QNAM was reset
in between, the reply might have been destroyed.
(This is only speculation based on the backtrace)