Fix null deref in sensors 2.0 VTS teardown

void SensorsHidlEnvironmentV2_0::HidlTearDown() {

    mStopThread = true;

    if (mEventQueueFlag != nullptr) {

        // Wake up the event queue so the poll thread can exit

        mEventQueueFlag->wake(asBaseType(EventQueueFlagBits::READ_AND_PROCESS));

        if (mPollThread.joinable()) {

            mPollThread.join();

        }

        EventFlag::deleteEventFlag(&mEventQueueFlag);

    }

}

void SensorsHidlEnvironmentV2_0::startPollingThread() {

    mStopThread = false;