D18558: Fix "QIODevice::read" warnings.

[David Faure]

dfaure created this revision.
dfaure added reviewers: dvratil, sengels.
Herald added a project: KDE PIM.
dfaure requested review of this revision.

REVISION SUMMARY
  Calling readAll() after close() is just nonsense, QIODevice checks
  the open status before reading, and warns (and returns -1) if closed.
  
  False warnings make people start thinking in the wrong direction when
  debugging an actual issue.

TEST PLAN
  `ctest --verbose -R sqlite-tagtest` no longer has all these warnings.

REPOSITORY
  R165 Akonadi

BRANCH
  2019_qiodevice_read (branched from Applications/18.12)

REVISION DETAIL
  https://phabricator.kde.org/D18558

AFFECTED FILES
  src/core/connection.cpp

To: dfaure, dvratil, sengels
Cc: kde-pim, dvasin, rodsevich, winterz, vkrause, mlaurent, knauss, dvratil
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