Clean up version test.

Clean up version test.

!(nfhdr->nflog_version) is equivalent to (nfhdr->nflog_version == 0).

That will evaluate to 1 if nfhdr->nflog_version is 0 and to 0 otherwise.

So !(nfhdr->nflog_version) == 0 is equivalent to nfhdr->nflog_version != 0,
but 1) it's more obvious what it means and 2) compilers don't point out
that it may not mean what you intended.