WT-6710 added metadata check in the following if condition, which dereferences cbt without a NULL check. There are potential callers that pass a NULL argument for cbt, making this a real bug that can cause a seg-fault.

    /* Don't check if transaction isolation is not snapshot or the table is metadata. */
    if (txn->isolation != WT_ISO_SNAPSHOT || WT_IS_METADATA(cbt->dhandle))
        return (0);