QCompleter: fix crash when setting the same model twice
Found when retesting the testcase completer.zip from QTBUG-54642
Pick-to: 6.3 6.2 5.15 Change-Id: Id84eefeb3a33dc6d790cfa23755352381cc097a9 Reviewed-by: Volker Hilsheimer volker.hilsheimer@qt.io (cherry picked from commit 7382e573)