Correctly set previous session state to enable cleaner session

During DriveHandler shutdown (in case of critical failure) the check if (statesRequiringCleaner.count(m_sessionState)) is never true, because m_sessionState has been reset in previous steps (in processSigChild() or processTimeout()). Therefore the cleaner session is never run at the end of terminated session.

It can be fixed by using the correct variable, m_previousState, in this check.