Coverity reported a dereference after a NULL check in dbuf_verify(). If
`dn` is `NULL`, we can just assume that !dn->dn_free_txg, so we change
`!dn->dn_free_txg` to `(dn == NULL || !dn->dn_free_txg)`.
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reported-by: Coverity (CID-992298)
Closes#14619