rtlogFlush: Potential null pointer dereference

[ColinIanKing]

Function rtlogFlush() in vboxguest/common/log/log.c performs a paronoid check:

    /*
     * If the ring buffer is active, the other destinations are only written
     * to when the ring buffer is flushed by RTLogFlush().
     */
    if (   (pLogger->fDestFlags & RTLOGDEST_RINGBUF)
        && pLogger->pInt
        && pLogger->pInt->pszRingBuf /* paraoia */)
    {
        rtLogRingBufWrite(pLogger->pInt, pLogger->achScratch, pLogger->offScratch);
        pLogger->offScratch = 0; /* empty the buffer. */
    }
    else ... 

This implies that pLogger->pInt could be NULL (even if this is an unlikely paranoid case). In the else path we have the following code that dereferences pLogginer->pInt and hence we may have a NULL pointer deference:

        if (pLogger->pInt->pfnFlush)
            pLogger->pInt->pfnFlush(pLogger);

I guess this is unlikely, but I think it would be good to correctly sanity check this for this corner case.