Camera2: Fix off-by-one error in ZSL selection.

    if (mZslQueueTail != mZslQueueHead) {

        CameraMetadata request;

        size_t index = mZslQueueTail;

        while (request.isEmpty() && index != mZslQueueHead) {

        while (index != mZslQueueHead) {

            if (!mZslQueue[index].frame.isEmpty()) {

                request = mZslQueue[index].frame;

                break;

            }

            index = (index + 1) % kZslBufferDepth;

        }

        if (request.isEmpty()) {

        if (index == mZslQueueHead) {

            ALOGV("%s: ZSL queue has no valid frames to send yet.",

                  __FUNCTION__);

            return NOT_ENOUGH_DATA;