D9190: TransferJob: workaround for when the readChannelFinished has already been emitted

[Albert Astals Cid]

aacid added reviewers: dfaure, apol, albertvaka.

REPOSITORY
  R241 KIO

REVISION DETAIL
  https://phabricator.kde.org/D9190

To: aacid, dfaure, apol, albertvaka
Cc: #frameworks