Code assist: Fix memory leak in completion

One fix is in the runner, which is part of the completion
engine itself. The other is specific for C++.

Task-number: QTCREATORBUG-5947
Change-Id: Ib8fff1eb5adad1ffb2a11da66c50b545e1457df8
Reviewed-on: http://codereview.qt.nokia.com/4355


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@nokia.com>
(cherry picked from commit e15443e9)