[no subject]

    fsck_ref_report(..., FSCK_MSG_BAD_REF_CONTENT, NULL);
    report(..., FSCK_MSG_DUPLICATE_ENTRIES, NULL);

So, we should check whether `fmt` is NULL in the `fsck_vreport`
function to make sure that if FSCK message is good enough to explain
what happens, we should not pass any message.

> Thanks.

Thanks,
Jialuo