bug#16161: 24.3.50; gnutls connections from Gnus no longer work after r115526

[Eli Zaretskii]

> From: Andreas Schwab <schwab@suse.de>
> Date: Mon, 16 Dec 2013 17:16:12 +0100
> Cc: Paul Eggert <eggert@gnu.org>, 16161@debbugs.gnu.org
> 
> Ted Zlatanov <tzz@lifelogs.com> writes:
> 
> > I've changed the `CHECK_LIST_CONS' back to `Flistp'
> 
> Flistp never returns zero.

Indeed, so the original code was clearly in error, and that test never
worked as intended.

Why not use CHECK_LIST instead?  It does allow for nil, and is
slightly faster.