bug#22789: 25.1.50; In last master build https connections stop working

[Eli Zaretskii]

> From: Alain Schneble <a.s@realize.ch>
> Date: Sun, 28 Feb 2016 00:49:25 +0100
> Cc: "José L. Doménech"
>       <j_l_domenech@yahoo.com>, 22789@debbugs.gnu.org
> 
> Now, if I do...
> 
>         if (errno == 0)
>           errno = EAGAIN;
> 
> ...just after the call to set_errno above, guess what: It seems to work!

This must be conditioned on something that requires EAGAIN.  Otherwise
overriding the errno of zero sounds like a bad idea to me.

Why does the nchars == SOCKET_ERROR happen here at all, if winsock
returns with an error of zero?  Isn't that strange?