omx: prevent crash at MuxOMX::freeNode for already freed nodes

status_t MuxOMX::freeNode(node_id node) {

    Mutex::Autolock autoLock(mLock);

    // exit if we have already freed the node

    if (mNodeLocation.indexOfKey(node) < 0) {

        ALOGD("MuxOMX::freeNode: node %d seems to be released already --- ignoring.", node);

        return OK;

    }

    status_t err = getOMX_l(node)->freeNode(node);

    if (err != OK) {