Avoid overflows in MediaClock calculations

    if (mAnchorTimeRealUs != -1) {

        int64_t oldNowMediaUs =

            mAnchorTimeMediaUs + (nowUs - mAnchorTimeRealUs) * (double)mPlaybackRate;

        if (nowMediaUs < oldNowMediaUs + kAnchorFluctuationAllowedUs

                && nowMediaUs > oldNowMediaUs - kAnchorFluctuationAllowedUs) {

        // earlier, we ensured that the anchor times are non-negative and the

        // math to calculate the now/oldNow times stays non-negative.

        // by casting into uint64_t, we gain headroom to avoid any overflows at the upper end

        // when adding the fluctuation allowance.

        if ((uint64_t)nowMediaUs < (uint64_t)oldNowMediaUs + kAnchorFluctuationAllowedUs

                && (uint64_t)nowMediaUs + kAnchorFluctuationAllowedUs > (uint64_t)oldNowMediaUs) {

            return;

        }

    }