avoid sending pointer values in struct passwd

[Yuichiro NAITO]

Thanks for reviewing my patch.

> 2020/11/20 23:45、Peter Stuge <peter at stuge.se>のメール:
> 
> Yuichiro NAITO wrote:
>> Take a look at my GitHub pull request to see my patch.
>> 
>> https://github.com/openssh/openssh-portable/pull/216
> 
> I think the length at the beginning should be tied to the (number of?)
> members that are sent instead of the struct passwd size on either side.

OK.
I fixed to send number of struct passwd members at first in sshbuf_put_passwd().
And sshbuf_get_passwd() checks it.

> Also, adding passwd.fields seems to be an unrelated change, better
> placed in a separate commit.

It is used for FreeBSD sturct passwd.
I made a different commit and wrote about this in the message.
So I force pushed new branch to the same Pull Request.
Please refer to the new one.

> Finally, sshbuf_free_passwd() is added but never called. If it is not
> needed then I think it's better to not add it (yet).

sshbuf_free_passwd() is used for error case of sshbuf_get_passwd().
I think it is easy to read and worth to be remained.

—
Yuichiro NAITO
naito.yuichiro at gmail.com